Heh, I'm pretty sure the OP was talking about older models of Mac computers, the ones without the new Intel chip.
To the OP: I sync my iPhone to my Powerbook G4 but did all my modding on my Intel Mac ... I've had no problems using one box to modify the iPhone while keeping it synced with a separate one.
Do you have a buddy with one that you can borrow for a few hours? Once you get ssh and SFTP on there, you can do all your modding through ssh (Terminal)/FTP without having to run iPhoneInterface/etc again, which means you can do it from the PPC Mac.