Please don't buy a Vista computer yet. I got to play with a brand new Toshiba Satellite with an Intel Core processor and 1gb of RAM this week. It was pokey. In addition, Vista was annoying as this Mac commercial cutely demonstrates.
It's still possible to find WinXP computers (my friend David Castro at TigerDirect will be happy to help you - 888-358-4437). You can get a great deal on a WinXP computer and be confident that Microsoft will support it for the next seven years (until 2014). I wouldn't advise planning to keep your computer seven years - five is a more reasonable estimate for the life of hardware.
So, if you have to buy a computer right now, buy XP. Don't bother getting hardware that supposedly will allow you to upgrade to Vista - the hardware will change rapidly in the next two years because what's out there right now can't quite handle Vista.
I worked on a laptop recently that was built at the very start of the WinXP era. It's struggling. XP's hardware requirements have increased since it came out. I could only upgrade the laptop's RAM slightly, from 256mb to 384, which helped, but XP is still struggling on that computer.
