Having already expressed interest back in April (and not having posted anything here since *cough*), I've made up my mind and am going to participate, most likely with my Power Mac G4 1.6 GHz and PB3400c/200. The former runs fine aside from its flaky USB 2.0 PCI card - on which only one port works reliably -, while the latter probably won't see much use, but has at least gotten a minor upgrade via a Cardbus USB card - on which also only one port appears to work, for whatever weird reason. I will have to use a modern Windows PC at work, running proprietary database software, but elsewhere it'll be all PowerPC.
In addition, as more of a console gamer than a PC or Mac one, I'll also try to stick to PPC-based systems there, such as the GameCube/Wii or, while not as old, the PS3 (Cell is PPC-related, right?). Might be a good time to start another playthrough of Metroid Prime or Zelda: Wind Waker, I suppose!
(Edit: posted from said G4, btw; only wish this site was a bit more responsive in its browsers...)