Buy a new one. And get it from a local or small shop to get premium parts. There's a some good shops that'll custom-build machines on the web, ones that come to mind are Alienware
http://www.alienware.com, Computersuperstore
http://www.computersupersale.com/, or if you know anyone in Canada buy your stuff from Razor Computers (they build it with no extra cost and ship it free within Ontario)
http://www.razorcomputers.net/
I know what it's like to have a dinosaur computer. I got a p166 with 32MB and 2GB. I'm getting a new one soon, but I'm also emotionally bonded with this one it's probably turning into a mp3 server.