Plain and simple, Vista is better than xp. It will run fine as long as you have at least 1 GB of RAM, it runs really well with 2 GB, speaking from experience here.
Depends on your computer, but at the end of the day vista is so much better.
Id go with XP, vista is still having some problems with some of the nvidia cards, patches have come out and tried to fix the problems but I know on the higher end cards you get lower perfromance (check gamespy.com out on their building a vista machine and also freak cant remeber who else did something...), but I am not sure about Vista and the Nvidia models in macs. Vista for me right now is just to much of a hassle while XP had its bumps and seems to do just fine.
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Depends on your computer, but at the end of the day vista is so much better.