I'd avoid Vista if at all possible. All it is is an absolute resource pig(not that XP isn't, but Vista is much worse).
Get a machine with XP & 2gb of RAM(memory) and it will be many times as fast as the same machine with 4gb of RAM and Vista.
p.s. I recently bought an HP desktop with XP Pro and 4gb of RAM. I thought that 4gb was total overkill, but hollllllyyyyyyy shiiiiiiiit is it smoking fast.
All this crap with forcing new operating systems on people is nothing more than a scam between Micro$oft and Intel. The older stuff works just fine, but doesn't help their bottom line.