Hmmm well this thread is in need of a major update now that Conroe is out and AMD's prices have droped a LOT. I'll rewrite this guide when I get back from vacation (hence my recent inactivity) in about a month.
@Ahiga, an ATI graphics card will run fine on a nVIDIA motherboard, but if you want to go Crossfire you will have to get a Crossfire compatible motherboard.