First, stronger CPU. All the rendering is done in the CPU, just like all the graphical stuff is done in the CPU (PS and PSP), so a 3200 would be better, if you don't want to go with Opteron. Graphic card, no, an onboard graphic subsystem is for office computers where they run nothing more demanding than Word. I recommend nVidia, because ATI has some issues with Rhino, but that doesn't mean it will work badly for you. On that note, keep in mind that PCExpress slot is coming into market and the AGP slot is going to be phased out pretty soon. So you'll be stuck. Taking a PCExpress card means taking a m/board with that kind of slots.

-- erlik