Well ideally what I wanted to do with my next machine (with Vista x64) was have 4 gigs of RAM. I figured that it could really use 2 gigs right now, and although the other 2 would be a bit overkill atm, considering future games and native 64-bit apps (which presumably would take up more memory due to larger data widths) then 4 would probably be the best option. Unfortunately at the moment, RAM is expensive
I'm hoping that come February when I'm ready to build, there will be reasonably priced 4 gig dual channel packs available.