I'm guessing you'll be using the iMac in your signature?
I haven't run it on my i7 MBP yet but I used to run it on my 2.8 MBP via windows 7 and it ran great on higher settings. Im not sure how much the 512mb card helped compared to a 256 though. You might as well just give it a shot and see for yourself. Seems like you already own the game, bootcamp is free, and if you don't already have Windows 7 you can either *find* it somewhere or buy an OEM version for cheap.
If your debate is which version of Windows to get, then the answer is yes get Windows 7. Vista is a joke and I would say that W7 has now become superior to XP, and XP was great.