I'm into some of the emulators (ones that emulate "classic" computers or "classic" arcade games). If you hunt around the Web there are lots of folks taking a dedicated computer and installing it inside an arcade-style wooden frame, or an actual shell of a classic arcade machine, and then rigging joysticks and such through a USB port and basically setting up a multi-arcade machine.
This is something I've seriously wanted to do for a long time but never get around to it... but depending upon if you have an interest in that sort of thing and want a new project, it could be a cool/fun thing to have around and a P4 2.53GHz should run at least the classic stuff. Some newer emulated games require a lot more horsepower to play, but frankly my interests mainly go to the ones I remember as a kid anyway.
MAME is the arcade emulator... MESS is the computer system emulator. I've had lots of fun using both.