XP XP XP XP XP.
DON'T GET VISTA!!!#@@#!!!~
I'd at least wait until service pack 2 or 3 before making the switch... in XP I get a bluescreen of death maybe once every two months, in vista it was twice a day. I've had problems with drivers, games, and programs not working the way they're supposed to.. I've had features I used "taken away from me" due to the way vista has DRM built right into the very core of the OS, you simply cannot do a lot of the things you can do in XP. HD video is downsampled on vista, so no matter what, if you have vista, the high def movies you paid extra for, will look like crap so that you don't pirate them. I have two monitors, and I wanted to span my desktop across them so I could play games using more than one monitor... you can do this in XP, you can do this in f**ing windows 95... but you can't in vista and you never will be able to, since it "makes it easier for people to break copyrights" (though the pirates have always had a million ways around such things, it continues to inconvenience the regular people)... Or like, my sound card, since the manufacturer of my sound card doesn't feel like giving microsoft an extra $100,000 to recertify their driver, you can not and will never be able to use that sound card in vista. It works fine in xp and since their infrastructures are similar enough they wouldn't even have to rewrite the driver, it already works in vista... but since they won't pay ms its extortion fees they aren't allowed to sell their product.
Or, let's say you need to find help for a program that came out a year ago, well vista has thrown out its help system for no reason (ms's reasoning is that "we haven't improved it in a while so we'll get rid of it despite millions of programs depending on it")... so if you try to click the help button in any program you'll get a message saying you're out of luck... if you go to MS's website and track down the problem in their knowledgebase they give you a link where you can write to microsoft and request this help system that's been in every single operating system since windows 2.0, but they won't let you have it by default. Good job ms, you saved 200kb out of a 20GB install.
A number of very large companies have gone as far as to boycott vista, such as IBM for example, who even though they helped write parts of vista, they feel it is too buggy and unstable and so much of a failure that they will *never* use it on *any* of their corporate computers.
End rant.
Vista will slow your computer to a crawl and it's buggy and unstable. Stick with XP.