I found this article that may help:
Basically, this guy says that while there is an increase in performance if you increase your RAM from 1 gig to 2 gigs, there isn't a performance increase if you go from 2 gigs to 4 gigs.
So, 3 gigs should be fine.
Just so you know, my dh uses pcs and I think he's gone back to XP on all of them. I don't think I know anyone who actually likes using VISTA.