Well, with the core i7 it's either get 3 or get 6, right? And DDR 3 is still rather pricey, last I checked anyway.
The short answer is that three will be plenty but I would sure get 6 if I could possibly afford it - it's a bit of a luxury but over time you will be glad you have it. Having said that I don't think you would find a three gig system will be sluggish in any way - Vista is not that much of a memory hog really. That's a myth.
Frame rates with XP are essentially equal now. More RAM might help some with frames, according to Corsair.