Uh, if you continue to run XP, there is no need to buy a Mac to do it. Just keep running it on your current computer. Seems a bit simpler to me.

System requirements vary based on which version of Vista you want to run. For the all out version of it, figure needing 3 to 4GB of system memory, an $800 video card, and a quadruple-triplematched-fortified CPU.
Then you can run the things your Pentium 3 with 256MB of RAM and a 16MB video card can run at the same speed. Woot!
Of course, I am exaggerating a bit. In a few years, the hardware will be available to allow Vista based computers to perform at the same speed as current XP based computers do. but with a truckload of restrictions.
I can hardly wait to hear the screaming when someone tries to make a copy of thier just purchased HD movie and Vista says, sorry, you cannot do that. Between DRM and HDCP, you will be lucky to be able to burn anything on a CD/DVD.