First thing I have to say is don't buy Vista. Now I guess I'll tell you why. First off Vista uses A LOT of space. Using up so much space makes your computer run waaaaay slower. If you plan on running more than two programs at a time, stick with XP. When using Vista, be cautious of freezing, you'll need to be vigilant about only having about 4 programs open at once and 2 running a once.
Next reason not to get a computer with Vista is that most games don't play nice with Vista. If you do get a game to work together with Vista you'll most likely need to turn the graphics down all the way. If you find a game that does work nicely with Vista, I'd really like to know!
Lastly a couple other reasons Vista made me mad/sad. There are so many trials of programs on it. If you think “it's so nice to have Microsoft word on the computer” you suddenly have no Microsoft word and you need to buy a copy fast or you won't finish your report and you'll get fired : /. It asks you like 100 times if you're sure you want to install something or let it run. That's a couple reasons to boycott Vista.