As you have an iMac, I would normally suggest a FireWire interface over a USB one, however, USB will again do fine for your proposed needs at this stage
Something like the Lexicon Alpha would be nice - Lexicon Alpha Stereo USB Audio Interface | DV247
- It doesn't have MIDI, but if you can still hook your keyboard up via USB for playing software instruments in GarageBand, or plugging in the audio connections for recording sound from the keyboard itself, then that's no big deal
The Presonus AudioBox has a few more features - PreSonus AudioBox USB Audio Interface | DV247
- but is more expensive, so it does depend on budget too
What you will also need to consider is monitoring - how are you going to hear what you are playing / recording / producing..? Headphones will be ok, although limited of course. Your iMac's internal speakers won't produce the right sound either of course, so you will need some way of hooking up a set of monitors / speakers.
Again, this doesn't need to be hugely expensive at first - and you might well be able to plug into a sound system that you already have - but it is a additional consideration