There are no decent USB audio interfaces.
FIREWIRE IS A MUST for audio. Unless you are positive you will never be recording more than one instrument at a time you should consider a Firewire multi-channel audio device. Some examples of when you might need such a unit include recording a jam session utilizing more than two mics or maybe recording a drum kit with more than two mics (common). A multi-channel device will allow for much more flexibility in the long run.
All of the below audio manufacturers released NEW Firewire ONLY equipment in 2008.
Digidesign
Yamaha/Steinberg
MOTU
Lexicon
PreSonus
Apogee
M-Audio
TC Electronic
Focusrite
There are no USB to Firewire adapters. (Desperate aluminum Macbook owners are looking, but it can't be done.) USB can not "keep up" with Firewire.
The NEW (early 2009) white Macbook has Firewire. (and the new NVIDIA graphics.)
http://www.apple.com/macbook/white/