get the gigabyte sli motherboard first of all, below is a link to another post where i ranted about the asus sli
i have 2x 6800s in my system at home.. It really wasnt what i was expecting at all. the FPS you get out of it isn't worth the money (20-30fps difference between 6800 and 6600, still maxes out games like halflife and cs1.6 @ 100fps). Get 2 6600gts (100$ish a piece) .
32 bit windows only supports 3.8 gb of memory.. Secondly, once you get passed 2gb of ram windows gets weird on how it allocates memory and you run into problems.. Solution, get 2gb and wait for xp 64bit to come out before upgrading past 3. Unless you're doing some video editing or audio mastering (i hope not with pcie video cards) then you're never really going to use more than 2gb of ram..
so, for the price you save on 2gb of memory (150$ a stick) you can get 2x 6600gts and build your system sooner rather than later.