I've used a Dual G4 w/ OS 10.x for the last 7 years as our tethering 'puter in the studio. When shooting with the 1Ds1 & 1Ds2 transfers via firewire were fast and consistent. As noted in this thread, 1Ds3 + USB = dog slow on this kind of system. Last week I bought an iMac 24" with OS 10.5.3 and now USB transfers are just as fast as the firewire transfers. In a shoot today, I'd capture 10 - 20 shots at a burst and the system kept up. No faults, no dropped frames, no unhappy clients.
I'm now wishing for a wireless transfer rate with equal speed.
From what I've gleaned on various internet geek sites, the USB + 10.4 + PowerPC had slow transfers due to a high transfer protocol "overhead".
p.s. I was using EOS utility as the interface software, and opening everything up in DPP. My initial reactions are here