tried xp, vista with service packs and windows 7. I prefer windows 7 due to the sleek look and peformance boost that I got over vista. its not a huge performance boost, I might add, but you can defiently feel it being smoother.

I do not know the ins/outs of windows 7 and what was changed from vista, but like i said things just feel a little more snappy.
but before you go out and buy windows 7, have you tried optimizing vista and turning off some services that thrash hard drives and run in the background?

I turn off "windows search", this indexes every file on your computer that you use or change so when you search in windows vista/7, searches will be quick. I do not need this so i turn it off.
another one which I think might be windows 7 only, but you can check vista, is "super fetch". having this enabled causes a lot of hard drive thrashing (starting/stopping, etc) so i just turn it off.

check to see if there is no scheduled defragging taking place. you can check this in the windows defrag section and click schedule. and change your power options to performance mode.