i have a G4 350 (sawtooth) w/256 ram (also a G3 400 @ work w/128 ram) and i have no problmes w/playback on either one.
i dont think that your system needs to be upgraded, but if u feel the need, i would go with the MP or just the straight tower. Dont go w/the cube. You might want or need the PCI slots later down the road.
also, in my G4 towre @ home, i use ATA's for capture/playback. No problems there either! just use speed disk (norton) before capturing!
e-mail me if u have any more questions.
thedarkhalf@mac.com