Hey, I've finally got a gaming PC! I just build a PC with 4gigs of ram and a Nvidia GTX 260, and play Crysis, WOW, and TF2. Does anyone have any PC games I've been missing out on?

S.T.A.L.K.E.R.

You should get a Steam account and check out their weekly and special offers. For PC I'd recommend getting Empire Total War, Dawn of War 2 and Left 4 Dead (there's a lot more but those are just the more recent ones).

Since Assassins Creed is currently on offer on steam, you could always get that.

Question is way too broad.

Check out Tie Fighter, best PC game ever.

Left 4 Dead.

Bioshock if you have not played it already, quite cheap on steam.

Homeworld. 
Neverwinter Nights 2.
Diablo 2.
Diablo 3.
But really, try either of the Warhammer 40k series, or Total War series; I hear Sins of a Solar Empire is supposed to be pretty nifty.  If strategy isn't your thing, shoot for NN2.  If that's not it, just hold your horses until Chronicles of Riddick comes out.  Again.

"S.T.A.L.K.E.R."

Mount&Blade, Civ IV, Aquaria, Cave Story, Pirates!, Noitu Love 2. Yeah, you didn't need a new PC for all these.

Today I played Dangerous Highschool Girls in Trouble. It was great!

Oh you mean something that'll use your PC's power? Battleforge has been pretty good.

STALKER: Shadow of Chernobyl
Crysis Warhead
The Witcher
sins of a solar empire
Company of heroes
Empire: Total war
Dawn of War 2

Halo 2 PC, there are people who actually play it still.  Also Bioshock is a great game.

Fallout 3, BioShock, STALKER: Shadow of Chernobyl, Civilization IV. Just to name a few. There are so many good PC games. My current PC game is Empire: Total War.

If you want stuff that can't be done on any other platform, then I recommend Neverwinter Nights 2. (best D&D game in a long while)
If you are into Strategy, Galactic Civilizations 2 is one of the best 4X games out there, and Red Alert 3 is supposed to be good as well.
Of course, both of those games aren't exactly the best way to show off a good graphics card. In that case there are plenty of shooters that can satisfy. (Crysis, COD: WAW)

Lego Island.

Left 4 Dead ! thats what im getting anyway

F.E.A.R. 2: Project Origin? It's not the best game ever, but if you max out all the settings I'm sure it'd put to use your PC and look pretty. It got fairly good reviews, of course.

Edit: Mirror's Edge, if you don't get motion sickness easily.

Although a new PC isn't really needed for these, World of Goo and Audiosurf.  Hella cheap and hella fun.

Crysis or crysis warhead.

Depends on your genre of choice:

Anything on GOG.com or the Steam Top Sellers list I guess.

Tomb Raider Underworld
Unreal Tournament 3
Crysis + crysis warhead
Far Cry 2
Empire - total war
Red Alert 3
Command and conquer 3 - tiberium wars
command and conquer 3 - kanes wrath
settlers - empire at war
anno 1701
warhammer 40k - dawn of war II
world in conflict
neverwinter nights 2 + expansion
Mirror's Edge
Half life 2
Doom 3
quake 4
Prey
Enemy Territory - quake wars
Serious sam 2
Settlers 2 - 10th anniversery
Settlers - heritage of kings
Tomb Raider Legend
Prince Of persia 2
Age of Empires 3 + expansions
EVE-Online
Guild Wars
Worms 2
Switchball
Mass Effect
Dead space
Space Siege
Rollercoaster Tycoon 3 + expansions
Shadowgrounds
Fifa 09
MS Flight Simulator X
Vampire The Masquerade - Bloodlines
A Vampyre Story
Battlefield 2
Battlefield 2142
Ballance
Earth 2160
Spore + creature pack
Oblivion
Fallout 3
Bioshock
DIRT

The Witcher EE
Neverwinter Nights 2
Call of Duty 4 (get it also)
Orange Box (if you have not got it)
Left 4 Dead (PC version is a must)
Stalker: SoC
Crysis: Warhead
Fallout 3
Unreal Tournament 3
Noitu Love 2
World of Good
Audiosurf

Minesweeper

The original Dawn of War and it's expansions,mainly Winter Assualt.Dawn of War 2 could have been so much better if they wouldn't have stripped out so much of the content,also if it didn't have Windows Live because man that thing is awefull.Apart from that i would go with Unreal Tournament 3,CS:Source,Left 4 Dead,Call of Duty World at War,any of the Battlefield games,Garry's Mod(requires Source engine game to run) Age of Chivalry mod for Source engine,Red Orchestra,I-Fluid,and The Witcher Enhanced Edition. Those are about half the games i have on my PC,the other half being mods and some games you already mentioned.

EDIT:Also Spore is amazing,and the Strong Bad games are hilarious and awesome.

My top 3 from this list would have to be the original Dawn of War,Garry's Mod,and Spore.

Hope I helped:)

Dawn of War 2, yo. Also, The Witcher: Enhanced Edition. I think both are on Steam in the US of A.

The Witcher?

Big Rigs: Over the Road Racing.

"Big Rigs: Over the Road Racing."

Orange Box
Fallout 3
Far Cry 2
Red Alert 3

and do NOT buy on steam before you shop around, games on Steam are almost always more expensive than ordering physical copy over internet or buying from other digital stores. On weekend sales the steam games might be cheaper or, more ordinarily, equal in price to other outlets. For example, all games besides orange box is at about half the price on Direct2Drive compared to the pricing on steam.

Fallout 3, Oblivion. :D

"Every PC gamer should own The Orange Box. It's by far the best value in video games.  Imho "

In addition to the games already mentioned, I'd recommend checking out gog.com for their games.  They've got a bunch of great old games you can download for fairly cheap prices.  I also really recommend the new Sam and Max games. 

I just finished beating Neverwinter Nights 2 and that game was awesome. Fallout 3, if you don't have it for another system, is also a good choice. Just get steam and buy some good games off of there.