Remember the words of Thumper's Mom, Disolitude. If you don't have anything useful to say, don't say anything at all.

'least I think that's the quote.

Big Rigs! Trust me, its awesome!

I would suggest Warhammer Dawn of War 2. Its a squad based RPG, though it looks like a RTS. My favorite FPS's are Battlefield 2 (64 player madness)and Counter Strike: Source. All of these games should be playable on your rig.

S.T.A.L.K.E.R: Shadow of Chernobyl
The Orange Box
Half-Life
Quake Live (Awesome for a quick game!)
Left 4 Dead
Clive Barkers Undying
Audiosurf
Crayon Physics Deluxe
Max Payne 1/2
Sid Meier's Civilization IV
Fallout 3(Fallout 1 & 2)
The Path
Torchlight
Zeno Clash
The Witcher
Baulders Gate 1/2
Icewind Dale
Neverwinter Nights
Planescape: Torment
Deus Ex
Dreamfall
The Longest Journey
World of Goo
Dawn of War and expansions
Sins of a Solar Empire
X3
Freelancer
System Shock 2 (What Bioshock was made after, except infinitely better than Bioshock)
Total War Series
Oblivion
Morrowind
Doom 2/3
Diablo 1/2
Starcraft
Painkiller
Hearts of Iron 1/2/3

Nice suggestions everyone. Based on what ive seen I will most likely purchase Warcraft 3, Battlefield, and ages of empires.

Also, I might try Counter Strike Source at my friends house. It looks alright.

You guys might think i'm weird, but I am going to try Bus and train Simulator 2008 LOL.

 Not really, PC is cool because you can find lots of esoteric detailed sims and random crap like that.

Plants vs. Zombies

Some of the old games remain the best but if you are looking for RTS Civilization is hands down awesome, also Rome Total War, Age of Empires 3. Half Life 1 and 2 should run on your system as should the very good medal of honor games. No system would be complete though without system shock 1 and 2 and some good old school FALLOUT.

It may not be a concensus, but I think CS 1.6 is better than source and Age of Empires 2 (with conquerors expansion) is at least as good if not better than AOE3, and still has an active online community with plenty of games to be found 24 hours a day, so you should at least try these earlier games if you're interested in the franchises.

Based on your choises you may not be interested in Black and White, but it may be great if get into it, even if it's an old game.

Also, the PC is great for niche and older games you can always go back and try, so you might be interested in things like Populous or Dungeon Keeper, for example, or other simulation games since you are trying one.