I ran the release candidate of Vista on a machine that is now in a MAME cab. 2ghz Athlon XP, 512MB RAM, GeForce Ti4200. Its a solid MAME machine for all but CHD's. It took 4 minutes to boot Vista and was given 1's across the board in the Vista 1 to 5 system rating utility. This same machine running a tweaked version of XP gets to the front end in about 30 seconds.
Granted, I didn't have unnecessary stuff turned off in Vista, and I wasn't using it as a MAME machine. I was just using it to check Vista out. It might be better in the future, but right now there are no gains to be had buy using it, and you also need more robust hardware for the same results. For dedicated cab use, XP or 2000 is way better, and given the choice based on my current MAME hardware, I'd even take Windows 98 over Vista (its easily skinnable, easily stripped down and shelled, and also very reliable when you're not using it for anything more than a cab).