Are you referring to a 5.1 audio interface that connects to your MAC and is addressed by Final Cut Pro?
I understand that Soundtrack Pro 2 will have essentially a suround panner feature, but until you get the sound into Compressor, there is no way to divide each track into a discreet speaker coming directly out of FCP.
There are a few 5.1 and even 7.1 computer speaker setups you can purchase in the $400 range, but no guarantee they will work in FCP.