Terminal Pharmacy hit it on the head, mic wise. But I guess you already have one. Just make sure to buy a pre-amp for it or it will a) be quiet as fuck on your computer, and b) sound like shit because you'll have to turn up the volume. Musicians Friend.com has some suitable mic pre-amps for under $100.
Also, GarageBand isn't only processor intensive. It's mainly RAM intensive. GarageBand is allocated RAM when initially started but when you fill it up and it starts to fart, keep hitting "Okay" after it gives the "I farted" message. It's some complicated process that my friend tried to explain to me, but it dumps RAM from other applications that aren't used or being used presently and will allocate some more, it just takes a little bit. But sure, it could affect your processor a bit.
But anyways. GarageBand is really not so professional. But for free and standard on a Mac, any small shit that I want to do is perfect for it. Any pro shit, and I would probably use Pro Tools or Logic.
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