Hello, GOG.
Here's a short story: I saw Fallout 3 on sale on Steam for $25 last week, considered buying it, decided I should play Fallout 1 first, went to GOG, bought and downloaded it, thought it was amazing, finished it in 4 days.
Now, I'm looking for other great PC RPGs, a genre which I never really paid much attention to. I've played Diablo 2 (loved it), KOTOR (same as D2), Morrowind (boring) and Neverwinter Nights (more than boring). I enjoyed Fallout more than any of them, though.
So, I come to you asking for more great games in the same vein as Fallout. Fallout 2 is the obvious answer, but I think I'll save that one for later.
Also, I know I could just go to a site like Gamerankings.com and see the highest rated PC RPGs, but I like to see some personal input before buying a game.

Why skip fallout 2? I would go for it, my favorite pc game ever hands down. Maybe try to find a copy of Wasteland?

Okay, so you want a good PC RPG... but NOT Fallout 2... in the same vein as Fallout 2.
Why not try Arcanum: Of Steamworks and Magica Obscura?
http://www.amazon.com/Arcanum-Steamworks-Magick-Obscura-Pc/dp/B00004TTHT/ref=sr_1_1?ie=UTF8&s=software&qid=1247088465&sr=8-1
It's a game produced by Troika, by some of the same people who worked on the Fallout series. It's a much bigger game than Fallout or Fallout 2, and there's so much material in it that I can't even really describe it. It can be played in turn-based mode (complete with Action Points) or in real-time mode, and I found it fun enough to play through it all twice (once with a human diplomatic guy and once with a big, dumb ogre warrior who couldn't speak properly and who suprised everybody by being the Chosen One).
The game is set in steampunk-y world where there is a running conflict between the forces of magic (that which alters the laws of nature) and the forces of technology (that which controls and exploits the forces of nature). You're dropped out of the sky (literally) into this world as a person who has unexpectedly become the Chosen One, a hero destined to save the world from a powerful evil.
The game has its flaws; it's really hard to play certain kinds of characters, there's a ton of combat that you can't avoid, and fire elementals will literally melt all your equipment before you can say "What the hell, man!? That was expensive!" But I think it's a great game anyway, and I think you'll find it addicting in spite of its flaws in much the same way that Fallout is.
EDIT: Don't take my preferences as gospel truth, though. I enjoyed Morrowind AND Neverwinter Nights.

I highly recommend Vampire the masquarde Bloodlines. It's a little rough around the edges but as an RPG its fantastic. Just make sure you patch it at least to latest offical version if not get the unoffical bug fix patches as well.

I would second this but if you really want to play more RPG games I would suggest the classic DnD rpgs people always loved like Baldur's Gate 2.
If you get Vampire, you must patch it no matter what since there will be game breaking bugs that will ruin the game experience. I remembered I had to use console to get past this one part in the game, ugh!

The Holy Trinity is composed Fallout, Arcanum and Planescape: Torment. Since you already played one you might consider these other two.
Mask of the Betrayer is also excellent, and probably one of the best CRPG to come out these last years. Despite being an expansion to Neverwinter Nights 2, an overal mediocre game, it is a completely different kind of animal. You do not even need to touch the NWN2 campaign at all.
And here are some games that deserve an honourable mention, listed with no real order:
Fallout 2
Vampire: Bloodlines
The Temple of Elemental Evil
Wizardry 8
The Witcher
Divine Divinity
Baldur's Gate 2
Icewind Dale
Icewind Dale 2
Daggerfall
Morrowind
NetHack
Diablo
Diablo 2
Gothic
Gothic 2
These aren't really CRPG but have enough RPG elements, as well a being cool games, to deserve being mentionned:
Deus Ex
System Shock 2
Jagged Alliance 2
King of Dragon Pass
The lists are non-exhaustive but this is what came to my mind when I read your request.

Try Gothic 2 + Night of the Raven. One of the greatest games I've ever played.
http://www.gog.com/en/gamecard/gothic_2_gold_edition
That''s value for money. You can't go wrong with this one.

Alright, thanks for the suggestions. I'll see if I can find Arcanum, Gothic and Vampire. I'll definitely add the others to my "must check out" list.
Thanks again :)

If you liked Diablo 2, then you WILL absolutely LOVE Sacred Gold. More classes, more races, more EVERYTHING. Oh and you can ride a horse (except for the dwarf for some odd reason).

You can't go wrong with this suggestion probably the best one of all some great titles.

The best game to play after Fallout is more Fallout :).
The true beauty of the game is trying different character builds and seeing what plays out differently. The options are so diverse !:)
For example, I'm almost finished my current campaign (as a weakling doctor/scientist build) and I am already thinking about a new build.. for a Cowboy! :)
The plan: High strength/stamina, low charisma (he is gruff!), takes the "bonus to single handed weapons" trait.. and I will tag small guns, unarmed (bar room brawler!) and gambling maybe (role playing, you know..) He will only use single handed guns.
Should be a lot of fun !
Fallout. The possabilities are endless.

Imo Vampire the Masquarade Redemption is better than Bloodlines.

I just downloaded Fallout. Loved the third, and I have been trying to find other old RPGs to play (I have only recently begun to like RPGs and I'm hoping to try some of the old classics). Is it worth getting Baldur's Gate, its expansion, Baldur's Gate 2 and expansion, Icewind Dale and expansion, and Icewind Dale 2 for $70 from Amazon? Also, does anyone know where I can find a cheap copy of Planescape: Torment, because it sounds really good.

It's sort of worth it but if you can find an Infinity Engine games collection that includes Planescape Torment along with the other games you listed get that instead. Planescape alone goes for like $50 US on eBay and the like...