I´m using Pro Tools on a Powerbook 1.5 GHz G4. It runs smoothly.
But in the end it really depends on your needs. Do you want to run dozens of heavy effects (reverbs, mastering tools etc.), or just want basic tools (eq, compressor etc.).. If the first is true you will hit a wall. If the second comply to your needs a 1.8 will be more than enough.
