I think moxnix has a good idea with building something based on the "Pentium M" series unless heat isn't going to be an issue. The Prescott core P4's run extremely hot and the 130nm core's put out alot of heat also. The Pentium M's have a better IPC than the P4. A Mobile Athlon XP would even be a good choice if heat is a concern. The Athlon64's are a little more pricey but run cooler than the latest P4's also.
P4's and Athlon 64's do have good thermal protection / clock throttling for hot environments tho.
Either way you go a Geforce 5700 Ultra or Radeon 9800 Pro card for graphics would be a decent budget choice and the Audigy 2 as someone has already mentioned for sound.
Keep us posted...sounds like a cool project !
