Vista is turning out to be the Windows ME of this decade...wait...Windows ME was this decade...ok...Windows Vista is turning out to be ANOTHER Windows ME of this decade.
WinME came out in 2000, so it was the last year of previous decade.
Vista is video card hog, so you need a good video card to play pc games or even have all the eye candy. XP is still the best for gaming, as it's using less resources and has better driver support. I would recommend waiting until SP1 for Vista comes out (expect beta to be out by November 12).
I tried Vista on 1.13 GHz Dell laptop and it was running fine, but when tried to play dvd movie (with Cyberlink PowerDVD), it would play in window only. Full screen would basically stall. Reinstalled XP and movies played without any hickups.
I'm not eager to use this os for another year or two (when more DirectX 10 games start coming out). There is talk about new os by 2009/2010, so I may skip this altogether and go for what's next.
I've been happy with XP since day one. It run almost two months at first withou reboot. As I install XP on many laptops and then have to update, it bothers me to see that the updates at least double the size of XP installation. Why so much crap??? If you plan on using XP just for Mame, install it with SP-2 and don't do any updates, as they will not affect playability in Mame.
If you have Vista Basic already, I would wait until SP-1 comes out and by an upgrade, so you will save some money and use Vista on your main computer (probably still dual boot).