Re: MarkIII tethered speed
I get a little less than three seconds shooting a 5D raw file into a 2.4 GHZ MacBook with 4GB RAM. That's using EOS Utility shooting into Bridge with file being written to a network drive over gigabit ethernet. The same setup with a 1.6GHZ G4 Powerbook was about 4 seconds. I have and still use a dual 1.8 G5 for editing. I don't use it for tethered shooting but I would expect it to fall between those two Powerbooks in terms of speed. On other tasks the G5 is considerably faster than the G4 Powerbook.
USB (if that's what you're using... MKIII is firewire maybe?) is picky about other things on the same bus. If you have some other USB device on the computer it might be slowing down the camera's performance. When I'm shooting tethered, the only other USB device on the computer is a keyboard/mouse.