I recently bought a high end pc, 6600 quadcore, 2 gig DDR3 mem and Geforce 8800 GTX Ultra, bit I kept my mem that low because XP can't handle more, unless I get xp 64 bits which has the same problems vista 64 bit recently has (drivers, drivers, application problems and... drivers).
So I stick to xp until Vista is a little older and more adult.
Reasons to get Vista: 64 bits, so alot more memory build in possibility, DirectX 10 (nice to see that little more FX) and Basicly because eventually it will run games faster as promised (in a year or so that is)
Reasons not to get it: still is in a baby state, haven't learn how to walk yet and well drivers and software support isn't that great yet, but will be eventually, just like with xp when it first came out.
So if you really want to use your 4 gig mem and really want to run directx 10... get vista (maybe in duel boot so you can run old games and software on xp still), if not, just wait a little like I do