I just recently built my new rig. I went with a 7900GS just because i had already blown most of my money on the motherboard. I had a certain amount of money set aside for the motherboard and video together. So it would have been
Gigabyte GA-965P-DS3 with a 8800 640 MB. (i don't really see the point of a 320. Unless you really don't game with really high resolutions and textures or your strapped for cash. and it is 80$ or so more. If your gonna spend a ton of money on a video card splurge some more and use far more of the potential of the core.. I don't mean the GTX there is a point where you are paying for a pointless premium. )
Asus P5WDG2 WS Professional with a 9700GS.
I made the 2nd choice and have not been disappointed. That card can handle all the games i play
Fear Combat -Almost full settings
Wolfestein: Enemy Territory (and soon to be Quake wars as well)
I just finished playing Halo 2 (i had it on medium and i got headaches from it rendering slowly so i went to lower details.And it'* on XP! I couldn't probably BARELY run it on vista.)
Call Of Duty 1+2
Battlefield 2142 (the demo anyway! LOL)
of course the occasional round of quake 3 with the bots and the original Unreal Tourney.
and i only tried it on UT2003 cuz i never play it.
In the end i know i will get an 8800 but not now, They will have to drop in price. Especially seeing as there are no directx10 games and i don't have vista. (and will NOT get it. I had the beta.....'nuff said.) Now i can't say i know anything about the games you want to play so i can't say how the 7900 will fare but heck. Most video cards now a days have so much power now, So long as you spend about 130$ (for the one i bought) or more you really can't go wrong. However that is MHO.