| I'd go with XP, if you can get it.
My experience with Vista hasn't been awful, but on every system I've tested it on it's been a total resource hog (something reportedly not as bad in W7). If games are your thing, DirectX can be installed on XP, and many publishers seem to be still designing with DX9 and XP compatibility in mind (though that may change).
However, I'd agree with everyone who says to wait for W7, if possible. I've heard excellent things about it, though I haven't tried it as of yet. | 
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