depends on whether or not you want a 64bit OS

vista64 > xp64
xp > vista

you catch my drift?

i'm not sure what problems there are with vista other than usability issues, memory issues aren't really a problem, since memory is pretty cheap these days.
you wont be able to find a lot of things in vista (assuming you knew where a lot of things were) and i think a few programs stop working if you turn aero off (go figure....)

alternatively you could wait for windows 7, i dont know when it's due for release

maya2008 runs fine on both, but my bet is, unless you have lots of ram i would stick with XP

but after the initial kick in the ball from vista's interface it's not soooo bad... although a lot of things i dont think i'll ever get used to...