Crysis if you want something to slap your computer around and call it a girl (especialy in dx10 mode).
Supreme Commander, great massive scale air/land/sea futuristic RTS. This one can make full use of a quad core cpu if i remember right, and give your video card a decent workout too.
Empire Total War. Big thread on it in the other games forum.
DCS Black Shark, high fidelity state of the art ka-50 sim, but has starforce (and so I won't be getting it
)
Dead Space, horror shooter on a space ship, haven't tried this one yet but its suppost to be realy good
Far Cry 2 had good and bad points
Spore, dunno if you would like this one or not, kinda open sandbox, i found it dull after a bit
X3 Reunion & X3 Teran Conflict. Both are realy awsome space games where you can build a trading empire (including stations and complexes of linked stations), explore and fight in any ship found in the game (you can theoreticly pilot any ship in the game). Jaw droping graphics and detail, but don't expect much of a story (its played for the sandbox aspect and large modding/scripting community).
Mass Effect, good shooter rpg hybrid, but kind of repeditive in areas.
Other ones you could look into, assasins creed, gears of war, gta 4, saints row 2, the witcher enhanced edition, warhammer 40k dawn of war I & II, rail simulator, trainz simulator 2009, the orange box, company of heroes, crysis warhead, world in conflict.
Ooo almost forgot, get ArmA so you can come play with the clowns (once they are done taking their ArmA break anyhow)