Well, the quality of your recorded signal depends on the weakest link of the chain. First, youl have to ask yourself what kind of sound and quality you are after.
If you just want to make some scratch tracks for future reference and stuff, you are ready to go (given that an Axe is quality by itself). Just buy a 1/4" to 1/8" adapter, plug your Axe output to your Mac's mic input and you're good to go. However if you want more quality you need to consider that onboard soundcards are not that great for professional recording purposes. The converters AD/DA on these generally suck big time (although Macs are better than PCs on this), and usually don't provide prepared inputs (i.e. balanced, XLR, etcetera). That's where recording interfaces play a major role. The better the quality and features the more expensive they get, even at exponential rate.
There are lots of alternatives on interfaces, USB or Firewire, lots or few inputs, with onboard effects or plain, you name it. You'll have to decide want kind of features you want and how much you want to spend. Go to the manufacturers websites and do a little research. A interfaces I can think off that you could plug into your Mac (going from cheap to expensive):
- M-Audio Fast Track USB or Pro
- Lexicon Omega
- M-Audio Mobile Pre
- Line6 Toneport UX1, UX2, UX8, KB37 or POD Studio
- Apogee Duet
- RME Fireface 400 or 800
- MOTU Ultralite MKIII
