For gaming, I'm reasonably sure the bottleneck for current iMacs is the video card.
I agree. I think with EA move to start developing for Mac, and the fact that ID is developing a Gaming engine that will work on Mac, that this will be indicative of apple needing to step up their Graphic card offers.
I'm not saying these are going to be that cards they wold offer but i believe that this wold a a wise choice:
Option one:
17" with 8600GT 256M stock
20" with 8600GT 256M stock or optional upgrade to 8600GTS 256M
24" with 8600GT 256M stock or optional upgrade to 8600GTS 256M
Option two:
17" with 8600GT 256M stock
20" with 8600GT 256M stock or optional upgrade 8600GTS 256M
24" with 8600GT 256M stock or 8600GTS 256M stock or 8800GTS 320M optional upgrade
I do not see Apple using any ATI cards, I'm sorry but ATI has pure crap for cards at the moment and it would be stupid for Apple to offer a inferior card while they are pushing for developer to make games for the Mac. Nvidia is the superior card maker.
The reason why I choose those card are because of power consumption. They consume a lot less power compared to the 8800 GTX and Ultra. Even the 8800 GTS with 640M consumes to much power.