It's not something I can build, but I'd love to own one of those "quad core" G5s. I have absolutely no interest in Intel-based Macs. They're yet another x86 machine running yet another slow and bloated OS. But the old PowerPC rigs? Now those are cool as hell.
I've already maxed out and overclocked a dual G4 rig, but one of these days I'd love to crank up the air conditioning and just go to town on a quad core G5. I mean, what's not to like? You've got all of the easy to use modern stuff: PCIe, SATA, and cheap DDR2. But it's all on a totally exotic motherboard hosting a pair of liquid cooled, high clocked, dual-core PowerPC 970MPs. Bitchin!
Unfortunately, the only G5s I ever come across at garage sales are the single CPU 1.6 and 1.8GHz machines. My overclocked (1.5GHz) dual G4 runs circles around those things.