Agent,
I run Vista Ultmitate no problem on a dekstop machine that is coming up for two years old since last major configuration.
My current config is Core2Duo 2.4ghz, but to make vista run sweet upgrade to at least 4gb of RAM as vista is rather memory hungry.
I would recommend looking at a Core2Quad as these systems can be bought very cheaply nowadays and are more than capable of running Windows Vista and Windows 7 (The next release). Memory is relatively cheap nowadays so adding memory after you have bought a package is not going to break the bank and is simple to do.
If your going to upgrade memory on a machine DO NOT buy the memory from somewhere like PC-World (in the UK) as you will pay heavily over the odds for it - use a specialist vendor such as Crucial (
Memory upgrades, flash media, and usb storage at Crucial.com)
Cheers
Jof