However good Vista is meant to be, it's another big step for MS, so I'd wait at least six months after launch before buying it, and that's over a year away!
XP Pro SP2 is good for me, stable, understood by loads of people etc. Unless there's a feature you must have that only exists in Vista, then buy now, IMHO.
There will be loads of hype,
PR tosh about how bleeding edge Vista is, but it's just an OS, and unlikely to halve your workload.