Tho as you mentioned, SLI/XFire support is more common/cheaper on matching board/cpu/gpu than mixed.
FWIW: My new build
http://secure.newegg.com/WishList/PublicWishDetail.aspx?WishListNumber=19733245
Went with Intel and i3 as I have guaranteed high quality CPU upgrade option in a couple years, whereas AMD I might need to ditch the AM3+ socket to get a good 8 core (or put up with lower performing piledriver, faked 8-core).
Went with Sapphire Radeon (AMD) 6850 as it is hands down the best bang for your buck gfx card at the time. NVidia's same capability cards are 33% or $50 more (at higher end, I think NVidia holds advantage).
Since my board doesn't have the slots to even consider xfire/sli.... and I plan on keeping it for a while, that wasn't really a concern for me.
Build cost is ~$700 though that includes over $100 in gaming accessories (mouse + left hand control) that really can be skimped on at first build (you could start with cheap USB keyboard mouse really)