Like many of you (I suspect), I already have much of Interplays catalog. For me, many of them are in their Macintosh incarnations - I have a decent PC these days, still have an older Mac I can play these on if I really wanted to, and a new Intel mac that may or may not run them well. I hardly ever actually play anymore (don't have time/it's just not as engrossing in single-player now that I'm growed-up and married) but this beta deal is too good to pass up, but don't want to waste my time money. So, below I'll list what I already have and my notes/thoughts on them to give some idea on what I might like and...not like, and solicit suggestions/votes on what my (hopefully first of many) acquisitions from GOG should be. (P.S. I looked through the codemasters catalog, nothing looked interesting at all, but I won't dismiss suggestions out of hand.)
Descent 1 and 2 - Was amazing when they came out, but never really grabbed me like Doom did. Played through about 4 or 5 of the single player levels. Updated source ports available.
Descent 3 - Bought it mostly because it was shiny and new and would run on my then-new 400Mhz G4. It had a rather bad habit of crashing after an hour or two of play, corrupting my savegame, which sort of soured me on it. Don't think I'd be as entranced by the glowy pew-pew enough for it to be fun anymore.
Fallout 1 - I have the recently (? Within 5 years I think) re-released MacOSX version. My brother loved it and still gets excited if it's brought up. I know this is heresy, but I never could get into it... Perhaps I should get #2 a chance and if not, give it to my brother?
Freespace 1 and 2 - Already have them, what with the OSP and all. Not sure why anyone would buy these now?
Giants: Citizen Kabuto - Very much enjoyed the majority of this. (Mac version) I don't remember if I finished it, I think parts of it struggled on my old rig.
MDK 1 - Loved this one, though never finished it either..I wonder if this Classic MacOS version will run on an intel iMac? Perhaps I should get 2?
Redneck Rampage - Pointless
Shogo - Mac version again. Never figured out what the fuss was about. Big stompy robots and 'splosions should be fun, I remember everything just seemed floaty and "off"...
So, the ones I'm considering - kinda. MDK2, Fallout2. Jagged Alliance 2 - never played any of the JA's, but they are spoken of often, in a "amazing, in spite of/because of the abuse" way. Should I even consider it? Also, Sacrifice I know has a small but fanatical following...Die by the Sword I remember as being unique, and trying something different. (There was a demo, IIRC - 1:1 mouse to sword movement?)
Please, pros and cons both, remember I'm won't be playing these with the help of nostalgia. A game has to really grab me, preferably something I can just play and not have to fiddle with. I loved the feel of STALKER, but messing w/ patches and mods and such just became too much work for me to enjoy it - the last game I finished was...HalfLife 2 Ep:2? Heck, *STILL* haven't finished Halflife1, Deus Ex took me like three years, and I still have the last boss bit in PoP:Twin Thrones/RivalSwords and like 1/2 of Okami waiting for me...

As far as I know, Fallout 2 is really - REALLY good.
It's on-par with modern games in all areas not graphics.

Prepare to face the Spanish Inquisition.
I found Fallout 1 better in terms of atmosphere, 2 is far too silly. But both are great, so it's worth giving them a chance.
The original JA2 is supposedly as good as tactical squad-based combat gets, with the possible exception of the original X-COM. Unfinished Business should be not far behind. So if you like that kind of stuff, why the heck not.
Sacrifice is pretty unique, and also quite difficult. It's still very much an RTS, but the different perspective and control method takes some getting used to.
MDK2 - I have the game, but only really played around with the demo. Fun, goofy action game, with a bunch of puzzle elements if you're playing with the Doc. I liked what I played of it, anyway.

The only game your talking about that is really keeping true to what the game is when the first came out is MDK
(also fallout but if you wearnt a big fan about fallout then fallout 2 will be just as much "meh")
MDK was and still is a fast paced 3rd person shooter. and MDK 2 keep it that way. Not only does it have upgrades graphics but it also brings in two new charecters for you to work with (the proffeser mr. i forgot his name, and the cybrog 4handed canine MAX)
when playing the main charecter you run around in true MDK style killing of aliens in black latex and a hand mounted (sometimes face mounted) chaingun sniper.
When playing the professor you have to use your brains to get yourself weapons (aka the famous use toaster with bread to shoot toast)
with max you can use 4 uzi's at the same time.
When you see the story from 3 diffrent people instead of just 1 it is a pretty sure bet that the game wont get boring.
i will give you 1 last advise if you do get it. Save often.
over to fallout 2
Fallout 1 was this adventure RPG and fallout 2 is the exact same. Fans if the series will love it cause it gives even more of the greatness that is fallout. but people who didnt find it that interesting will find fallout 1 with another story and a bit more stuff to do.
If you should by this. it really depends on your thought on the first game
I only tried jagged alliance once. but i found it like the game commandoes and i didnt really like that game so i played it in like 5 sec before closing it again.
Sacrifise and die by the sword are games i have not tried yet
My guess of your game of choise would be MDK 2 (it even gives a nice story of what happend in the first game and how it ended so you dont really have to play the first one)
Good luck

I am in a similar situation as you Scandalon.
Most of the Interplay that are worth buying I already have them or played them before(like Giants, Sacrifice, Descent 3, Fallout Tactics).
You should really try Sacrifice if you are into strategy games. Sacrifice is kind of unique in that the player is playing in 3rd person view & cn support his creatures from the rear with spells.
If you are playing it for the 1st time, go for James(one of the 5 gods to chose) all the way, as most of his missions are easier.

JA2 is one of my favourite games of all time. And this isn't just nostalgia talking; it's only been about a year since I first played it. It's brilliant.
My only complaint is that it's kind of ruined the original two XCOMs for me. Their strategic layer still holds up perfectly, but the tactical stuff feels quite clunky thanks to JA2.
(of course, I'm the same guy who found Silent Storm unplayably clunky thanks to JA2, and everyone else seemed to love that one - I think I might just have a low tolerance for clunk.)
I haven't played Unfinished Business though, so I can't vouch for that one. Weird how it's the only one available here.