Might as well go for Vista since you can get it preinstalled with a new computer. I like it and feel it is an improvement over XP.
As ae86_16v said make sure to get a good system with 2GB of ram and a decent video card.
BUT
If you have any peripheral devices (web cams, scanners, game controllers, printers, mice) make sure they are compatible with Vista.. although if they are from big name manufactures and are relatively new, they should work just fine.
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