I am merely looking for a piano replacement that will let them play with some synthesizer-like effects. But keyboard action is the key.

I took a further peek and evidently GarageBand can do most of the work on the software side. I have it on iLife 2006, which runs on the Cube. But for the keyboard, does it matter whether the thing has a usb connection? Or do the midi-usb converters work just fine?