Best audio Interface for garageband?

There is no one 'best' interface - there is a very wide range available, ranging from simple stereo audio in/out to complex ones which have microphone preamplifiers and allow you to record multiple channels simultaneously. For starters, have a look at this page by our resident Guru, HangTime:

http://www.bulletsandbones.com/GB/FAQPages/Interfaces.html

One additional comment, "Best" is defined by your needs, and you haven't told us what they are

to record up to two tracks at a time, yes.

Can you recommend an interface for my needs- mostly live performance of software keyboard instruments on my Powerbook (currently organized in RAX2) combined with the outputs of 1 or 2 additional hardware keyboards (so I'll need some mixing capabilities). Right now I'm using the decidedly inelegant (but adequate sounding) solution of just tapping the laptop's miniplug headphone output thru adapters into a Fender amp. I will also use the interface to input guitar to the computer. I don't mind paying a bit more for better build and sound quality if I need to. Thanks Dave

take a look at the fbox listed here:

http://www.bulletsandbones.com/GB/Interfaces.html