It's still better to have the use of the extra gig, in Vista at least, XP needs less (and I've heard some ppl say XP can slow down if you have more than 2 gig?)
Anyway, I get 2.9. out of my 4 gig in 32 bit Vista and XP. I've got an 8800 gts (640) but you're not the only one getting 2.8, I've seen that number mentioned here at least a few times on this topic