Need some new PC game...

So I have a beast PC (https://secure.newegg.ca/WishList/MySavedWishDetail.aspx?ID=16956072)So I'm wondering if there are any good games that came out recently (or older) that you guys would suggest. Currently I only play StarCraft II on my PC and would like some game to play when I don't want to play StarCraft. I also have installed Halo, Halo CE, Halo 2, CS1.6, CSS, and TF2. I don't want to install Origin or anything like that (have steam installed). I'm pretty much up for anything, just looking for a good game to play.

EDIT: Please no CoD (the only 1 I would get is CoD4 which I think I will). Also, preferably single player games. Thanks!

Mass Effect series, Deus Ex: Human Revolution, Dirt 3, I'm finding Assassin's Creed to be pretty good (although it is best played with a 360 controller).

I recommend King's Bounty: The Legend and King's Bounty: Crossworlds. Pretty much a thousand hours of fun if you are into RPGs and turn-based strategy!

The Witcher 2 is also a game that presents itself nicely on a powerful PC *and* is great fun to boot!

Head to steam and BUY the STALKER bundle, its cheap, and those are great games.

Also Amnesia is really good, vampire the masquerade: bloodlines is really good (but older) and both Dead space games.

So there is alot of games right now that is just up for the picking cheap :D

Really enjoying Renegade Ops at the moment. Came out this week.