rome total war is amazing, especialy for testing your (eVGA 6600 GT PCI-E) 10,000 fully featured troops on each side, charging towards eachother should blow your mind pretty thourolly
also the new expansion is coming out this fall (rome: total war barbarian invasion)
http://www.totalwar.com/community/rtwbi.htm (the official site)
http://www.gamershell.com/download_10207.shtml (the demo for BI)
(check out the night battle on the demo, amazing)
get medievil just beacuse it was one of the greatest turn based stratagy games i can think off (the same caliber as the civilzation games)
BF2 and HL2 are just a good games with some pretty nice graphics
some other games that come out soon that you could test your new computer on are:
stargate sg1 aliance (looks amazing)
Flatout (if you like racing games)
Qauke 4 (whenever that comes out)
Call of Duty 2 (the 1st was great)
Stubbs the zombie: rebel without a pulse (just beacuse its hilarious)