I have an original MacPro1,1 that I've added drive space, RAM and video to. It's served me perfectly well for the last five years, and I had no intention of replacing it until it broke. Until now. Because now it appears it won't run 10.8, which I want to run. MacPro3,1's are fairly commonly available, and they'l run 10.8 IIRC. But I really don't know much about the differences. Can some point me to a compare-and-contrast between the two?