I played GURPS for a number of years (probably the last 15 years?) The rules are very extensive, and strive for realism (whether they make it or not is a matter of some debate). What GURPS does well is amazing amounts of character customization and deadly combat. What GURPS doesn't do well is that combat tends to take a long time in the vanilla rules, and the rules aren't organized well (even in the same book). They also don't have many actual adventures, so if you're someone that enjoys pre-written material (modules), then you're converting it from another game system. I'm heavily invested in GURPS (75-100 books, plus additional PDF purchases), but the pace of the game was so slow and the amount of prep time I spent was unreal that ...

A few months ago I switched our group to Pathfinder because I was mostly running their material (their adventure paths are some of the best material I've ever run), and the time necessary to do conversions to GURPS took forever. It's based loosely on D&D 3.5e, and we've found it to be a lot more fun because the pace is faster. It's easier to pick up than GURPS, and for half my group, they just wanted to get in and beat stuff up anyways. The half that want the deeper story and character customization still get it. We get so much more accomplished each night than we did with GURPS.

That's my $0.02. They are both excellent systems. I've played quite a bit of D&D 4e with another group, and I just don't like it as much as either GURPS or Pathfinder. D&D 4e feels very MMO-ish, if that even makes sense (it just feels like I'm playing WoW or something). I LOVE the flexibility that GURPS has with character customization, but I'm incredibly pleased with the increase of pace by switching to Pathfinder. I don't think my group will be returning to GURPS any time soon.

I don't know what kind of RPG you have in mind but the kind I've seen people playing most often require quite a bit of work if you're inexperienced and want to host the actual game (as opposed to providing a place for people to meet).
It's usually fairly easy to join a game that's already set up but someone has to do the work of a game server of sorts (using MMORPGs as an analogy). Risk is the only game I know of the five but your likes and dislikes made me think of Full Metal Planet. It's a serious game though. Carcassone is nice but less heavy. It's an award-winning game which allows much manipulation since you make the map during the game. I'm guessing there's a bunch of games which play in the same way.
If you want a fun Risk-like you can play without too much thinking, maybe you could try History of the World or one of its clones.

Munchkin is great, lots of fun. One tip, the more people the better. Any version will do but I am partial to the Original D&D theme or the Pirates Booty. For great board games... Settlers and Carcasonne (sp?) are great games but not really in your theme breakdown. That said, I can almost guarentee you will like them. You dont need any of the expansions (but you will want them afterward) to get started.

I played GURPS long ago when it first came out. Basically P&P AD&D for all intents and purposes although the rules can apply to a much broader setting. I myself prefered Paladium for P&P games.

For a quick and dirty game, Loot! is fantastic. Granted it appeals to my pirtae side, but even if it were my little pony based its a great game. Its stupidly simple to learn, yet you will revisit your strategy to play 100s of times over.

I found this PDF of the Fallout PnP. If you want, I can put it in my dropbox for you.

It's Fallout A Post-Nuclear Role Playing Game version 2.0.

Board game version of Civilization is enormous and well made and costs a lot. I think a game can easily last for hours.

Not exactly sci-fi, but nevertheless I like Arkham Horror. There are a few disadvantages - it takes a bit of time to get the hang of it, there are are so many expansions that you could easily blow hundreds of dollars on them, you would have to host the games yourself since no one else can be bothered to learn absolutely everything - but overall it's great fun if you can persuade three or more friends to join you.

EDIT: Typo.

[url=http://en.wikipedia.org/wiki/Paranoia_(role-playing_game)]Paranoia[/url]
While I've only had experience with the 5th edition, it was the silly kind of Post Apocalyptic humour that is excellent for some quick games without too much continuity. 2004 version of Paranoia XP returns to the more traditional roots of a normal rpg, but also includes the ZAP version of the "silly" 5th edition.

Give it a shot if you can find it, it seems right up your alley.

If you're into a WWII setting, for a board game choice, Axis and Allies was always good fun back in the day.

As well, if you can find them, give a look at Traveller and Gamma World. Those were enjoyable games RPG as well.

+1 for Arkham Horror. Also you can try Caylus, which has no random element, it is only a matter of tactics, like chess.

By the way, brettspielwelt.de is German website for playing boardgames online, it should have English version too, IIRC, so you can try playing many games there without need to buy them. Yes, it is not as good as playing IRL, but there is no shortage of players.

From RPGs I can recommend only FFG's Warhammer 40k roleplay, which is not classic sci-fi, but I like that setting. The last one, Black Crusade, even contains rules for interacting between those four games and character transfer (they use different power level and have different focus). Though be sure to check their errata.

Arkham Horror is 1st Class , if you want a marathon Session try Twighlight Imperium , for quickfire sessions you cant go wrong with Dominion Card Game :)

Oh and as another poster mentioned Civ Board game is Epic , I know I have it ! :)

... and almost forgot War of the Ring is epic too ! ( though quite complicated )

Muchkin is great fun. Fantastic humor in that game. For a fantasy themed board game, take a look at Talisman:

[url=http://en.wikipedia.org/wiki/Talisman_(board_game]http://en.wikipedia.org/wiki/Talisman_(board_game[/url])

I prefer the second edition and its expansions, but that could be pretty expensive. Still, I will say it is the most enjoyable board game I have ever played.

First, there is the new Conquest of Nerath boardgame:

http://www.boardgamegeek.com/boardgame/92044/dungeons-dragons-conquest-of-nerath-board-game

It's a fantasy wargame in a D&D setting, if you have ever played risk you should be at home here, it's just a little more complex. Also, you could try Descent:

http://www.boardgamegeek.com/boardgame/17226/descent-journeys-in-the-dark

it's alike to a tabletop RPG experience, but the DM is actively competing against the other players instead of being a "master of ceremonies". And lastly, if you're looking for some giant mech-action:

http://www.boardgamegeek.com/boardgame/31759/classic-battletech-introductory-box-set

and a personal favorite of mine, much lighter than the aforementioned ones, but we usually have a lot of laughs with it:

http://www.boardgamegeek.com/boardgame/70323/king-of-tokyo

I've played quite a bit of GURPS (about 15 years or so) as well as 20 years of D&D. If you're new to tabletop RPG's and none of your group is really familiar with them I'd avoid GURPS. It's extremely complex and not something to start with.

To be honest, for beginners I'd say D&D 4e is probably your best bet. It's easy to understand and unlike 3e or 3.5e they realized the mistake of not testing the products against all the previous releases and not just the core books. Balance wise it works out very well. It does have a sense of that MMO party creation, but for someone that isn't a PnP veteran that's actually a good thing. It's easier for people to understand the concept of Defender, Striker (DPS), Controller, and Leader (Healer), than to look at the ability progression and try to figure out what role a character has.

Now if you have a veteran DM/GM then you don't have to worry about this, because they'll know how to tailer an encounter to the party makeup. But for published adventures (where all beginners really should start) are going to need a lot of modification for unconventional group makeups.

tl;dr - D&D 4e is probably your best bet for beginner tabletop RPG players.

Now for Board Games...

Munchkin is great for a bunch of friends getting together, drinking, and just hanging out. Can mix and match the sets fairly easily without too much thought (save for some expansions like the portal/dungeon one).

Descent Journeys in the Dark is like D&D without the role playing. Just pure tactical combat. It's fun but pay close attention to the rules. The smallest rule can make all the difference. (Like the one about only a single potion use per turn).

Everyone is going to mention games like Settlers, and while they can be fun they get a little basic after a while. Your mileage may vary.

RoboRally is another game I really enjoy. Robots killing each other in a factory.

Car Wars, while old is a good classic, but the rules can get a little complicated if I remember correctly.

The Battle Tech Classic box set is another great choice, and is very board game like if you don't try to build your own mechs but just use pre-generated ones.

If you like risk, I'd look into Axis and Allies. But keep in mind that it is going to be a long game.

Finally while I haven't played them personally I've heard a LOT of good things about the D&D board games (In fact I have the Drizzt one on order). Raveloft, Ashardalon, and the new Drizzt one especially. Conquest of Nerath is supposed to be fun, but I've heard it's more like Risk/Axis and Allies.

You can't go wrong with any of the old Milton Bradley style war gaming board games. Conquest of the Empire, Shogun, Axis and Allies, Fortress America, all great games.

Also, Risk 2210 and Godstorm were pleasant surprises and streamlined enough that you can get a couple of games going in a night.

Battletech Classic was great, but you'll need to be prepared for a long game even if you opt for a small battle instead of a full blown campaign.