I'm shopping around for my first Mac, probably a "pre-owned" dual 1GHz G4. I plan to use it as a digital recording studio.
You could probably get that much power in a Mac Book, Mac Mini, or an IMac. If you already have a monitor, keyboard and mouse, the Mini can get data to an external firewire drive at a good speed. The difference in the two models is really the burner, the higher priced one can burn DVDs. I've lived through all their architecture changes, and my small tip would be to go with an Intel Mac.