I'd add that if you buy a out-of-the-box system and sticking with OEM parts, Vista isn't too bad. Aesthetically it does look nice, it will run just fine, plus you're guaranteed .

If I were building my own rig (which I'll be doing soon) I'd personally go with XP64 just to keep the number of headaches down.

That said it's all personal preference and everyone has their anecdotal evidence to support their choice.