Vista, according to my husband, is a HUGE memory hog. I ran Sims 2 and all EPs/SPs on 2 GB with that and it lagged big time, upped it to 4 GB and it was a lot better.
Then I got Win 7. 2 GB does fine, I now have 3 GB (salvaged a memory card from my old computer).
I hate Vista. Was glad to say "hasta la Vista."