Well considering Doom 3 is one of the most CPU-GPU intensive games on the market right now, YES! You'd probably have to have the GeForce 6800 Ultra or Radeon X850GT for your Power Mac. I'm not even going to try it with my iMac G5...
Supposedly though, Mac is supposed to upgrade the Power Mac's, wondering if the new 7800GTX or Radeon X1800XT/XL is going to be released for Mac.