Runs OK on a G4 Dual 1GHz with ATI 9800. I can even squeak by with my 867MHz 12" G4 PB for a quick visit without moving around much. The little guy runs really hot while doing SL, though.

The ATI can only pull about 13 FPS even off a good cable modem connection. It's fine for chatting, walking around, and building, but don't plan on mixing it up with guys - and there are plenty of them - getting 50 FPS.

Performance sucks in crowds of more than ten or so, but that's SL's fault, not your Mac's.