XP Pro is older, so has less bugs but I think it also does not support dx10.
Vista is also getting games designed for Vista only, meaning that unless you have Vista it won't run them...
I'd say if you get Vista stick with Basic +SP1 upgrade. It's around £51.57 at the moment on scan (today only offer) which should allow you some of the benefits of Vista OS without the more irritating aspects of it (games and some pointless software)
Of course.. if you want it retail, triple the price, but it's your choice.
XP is good, but I don't really like it.
Of course: if you've got a slower system (less than 1-2gb RAM in this case, as you're not suggesting single core CPU) then stick with XP as Vista is more demanding.
Not sure about the systems though, as I've not had much experience with anything newer than a P3 and 384mb RAM (win 98 fyi) so I'm a little Out of Date with hardware 'n' stuff