[PeteMac]

Dnew, you mentioned that you just want to get the sound you're hearing into your Mac . . .
This is not as simple as it sounds.
The SM57 mic is an industry standard for mic'ing guitar amps - Get one, you will always need it.
A second mic (if your budget allows) could be a large diaphragm condenser. Better for vocals, room mic'ing or drum o/heads.
Just plugging straight into your Mac will not get you anywhere near the sound you're hearing from your amp - the pre-amp in the Mac just isn't up to the job. You need a reasonable pre-amp/interface.
Lots of 2 input USB interfaces on the market - most have half decent pre-amps built in. (Most also supply "phantom" power to power your condenser mic's.)
I use the Lexicon Lambda - it works very well with my iMac and GarageBand. It has 2 mic inputs, a direct instrument input as well as midi ins & outs (for hooking up a midi keyboard to use with the virtual drum kits and other instruments supplied in GarageBand.)

That's really the minimum gear you will need - but getting the right sound is more than gear.
Experimenting with the placement of the mic in front of (or behind !!!) your amp untill you get a sound you like can take a while. There are lots of on-line articles relating to this; do a little research and it can save you re-inventing the wheel .

The search for the perfect sound into your recorder is almost as never ending as the search for the perfect guitar tone !
You may have to compromise your ideal, and get a sound you can live with . . .


But once you start, there's no turning back :-)

Have fun.