What is the application?
Most likely go with a new (high Ghz, high FSB) Intel Dual Core P4 or Xeon and an Nvidia Quadro card or Wildcat Realizm or ATI FireGL, if you're running any sort of 3D program that I'm thinking of, like Maya or something along those lines.
Orrr, get a dual G5 dual core computer with Quadro, thats 4 logical processors. (if you dont want to spend money this wont be an option however, >$3000)
Systems like that are designed for workstation graphics.