I had Vista running on my gateway before with only 1 gig of ram, and it set on average around 60%-65% usage, and most games (Crysis, COD4, NFS Prostreet) they all would pretty much max out the ram. I saw a considerable performance boost stepping up from 1 gig of 533mhz ram to 2 gigs of 667mhz ram.
I didn't go through and kill all the processes that didn't need to be though. The extra bit of ram made it seem more responsive and bootup/shutdown times were a bit quicker as well.
Thats my 2 cents though, may not be the same in every case.