building a speedy build station

I have a rare opportunity to spec out a machine specifically for running InstaDMG builds. So, a MacPro. But which parts do I beef up to get the best performance? Is the speed most affected by disk I/O? Available ram? Processor? SOme arcane combination of the three that hits the sweet spot between cost and function? My current specs are a a quad core with 4gb of ram, a 640gb drive for OS, a terabyte drive for storage and 2 striped 320gb 10k rpm velociraptor drives for the build section. (those drives are actually pretty cheap, and are, as far as I can tell, the best/cheapest upgrade I can get). Is this the right route?