Orange box ( 5 great games for only $30 )
cryostasis ( if you like horror games with great atmosphere, great graphics,great story) but keep in mind is slow paced
Sins of solar empires and empires total war if you are into strategy games ( first one is massive space battles the other massive historical battles amd both have great graphics and a massive scale especially sins)
Race driver grid is a good driving game with good graphics
Mass effect,fallout 3 should cover you in the rpg genre ( although mass effect feels almost as an fps game)
Kings bounty turn based rpg ( i highly recommend this is very addictive)
trackmania nations ( free arcade online racing game with good graphics)
Finally i recommend penumbra games ( all 3 can be purchased for $5 on steam) if you like horror adventure games ( all 3games in the series are the scariest games i played and that includes silent hill)
Also i recommend Unreal tournament 3 is only $10 on steam and there are some great mods for it
finally keep in mind that one of the most impressive military sims is coming next week Arma II ( look in youtube for arm2 1500 A.I. battle and 150 planes air battle very impressive stuff that should give your pc a run for its money
)
To conclude i recommend one of the best fps games the last 3-4years STALKER series with excellent graphics that i can say after crysis is the best looking game