Newbie input device question

Got my first Mac three weeks ago(21.5" iMac) and I'm ready to check out Garage Band. What would you suggest as an audio input device for a newbie? I doubt I will do a lot of tracking on the computer, but I would love to be able to try it out. I'm a guitar player so that would be the main thing I want to plug in. Should I get a $40 USB device or spring for a Firewire one. I want something that will get the job done cheaply, but not frustrate and alienate me.

A USB interface would be fine. Edirol (Roland) make excellent ones at anything from budget to expensive prices.

If you're just playing a guitar, then you can't go wrong with a first generation Line 6 Pod. (though there are lots of other options)

It has Direct input so you can plug right into your computer's input jack, set a virtual amp model and go nuts. You can also use it to record other instruments as well if you hook up a microphone to the POD.

You can find the first gen ones on ebay for about $40-60.

but if you want to get a more professional setup then firewire inputs are all you should be considering.