Daniel Knight at LowEndMac has taken on quite the project:
A couple weeks ago I picked up 12 used Power Mac G5s for free, and I already had five at home, including one dual-core machine. One of these has no power supply (I bought a single CPU model for less than a used power supply for my dual processor G5 would have cost me). Some of the newly acquired ones have no RAM, no video cards, and/or no hard drives. But except for the three 1.6 GHz machines and the dual-core model, they all take the same memory. Further, I can readily swap drives and video cards, giving the fastest machines better configurations.
This article really made me miss the old Mac towers. I hope the upcoming Mac Pro is a return to form in terms of upgradability over time.