One of my best friends is a sound engineer and runs a G4, but added a huge honking extrenal HD. He had that mini tower for protools, but has recently jumped up to the version with more inputs/outputs with no issues at all.
He has a pretty sweet setup with a korg triton, korg karma, MPC4000XL, ect all hooked up through the new protools, and not have to switch from unit to unit. The G4 is plenty good, he just needed the larger external HD to store everything he works on from day to day.