Are you asking about a sound card? Mac people tend to like the Griffin iMic. Since you're getting a Behringer mixer, don't get the UCA202. Macs get confused. If your Mac Pro has an audio line input, you can use the Behringer USB audio codec output for the main audio to computer, and you can use an aux out to the Mac Pro line input for mix minus.

If that's not your question, let me know.