If it were me, I would wait a couple of months and get W7.
Its due out in Oct.
It is basically a upgraded Vista.
I have been running Vista since it came out with no problems at all.
I havent tried the RC of W7 but I hear that it is better and it is based on the same code.
I think the support for XP will begin to fade before long so you dont want to fall into that hole.
If you currently have either one I would run your progs with that for now but if you can wait then I would.