Theophany, the current iMacs have a 1066Mhz FSB - they are overclocked Penryns.
uzit, I got a 8800GS and it's worth every bit! If you're gonna do some gaming, I suggest you go for the 8800GS as the upgrade is worth it - Snow Leopard also takes advantage of the GPU, so the 8800GS is going to make you very happy
Currently I run these games on Bootcamp XP SP3, all on 19200x1200 and all details high with AA off:
Bioshock, Tomb Raider Underworld, Assassin's Creed, Left4Dead, Fallout 3 and C&C Red Alert 3.
Currently the 8800GS (8800M GTS I think) support DirectX 10 - the 9800GS (or 9800M GTS) are pretty much a rebadge, with minor improvements. The new Montevina iMacs are gonna run at the same Ghz, but with lower power consumption.
I can also assume that the new iMacs are gonna have DDR3 Ram, LED backlit LCD (comes at a premium) and the 9600/9800 GPU - maybe an aesthetic change to the stand (slanted like the ACD). If you don't live in the US, then the new iMacs are gonna cost more - in Australia the updated Macbooks cost around $500 extra due to the economy.
I just bought my first mac (iMac) 3 weeks ago and couldn't be happier!