I have an Apogee Duet firewire audio interface that gives me one stereo channel in and out. It works great for single track recording and overdubbing but is not very functional for dj’ing. A great feature available in OSX is the ability to create aggregate audio devices. What this means is that you can use both the external audio interface and the built-in audio device in your Mac at the same time. This allows you to cue on one channel (the built-in 1/8″ mini-jack output) and send out to the main outputs on another (in this case the Duet).
To accomplish this, open Audio MIDI Setup under Applications->Utilities. In the Audio MIDI Setup menu bar, select Audio->Open Aggregate Device Editor. Click the + button to create a new aggregate device and then in the Structure window select which devices you want to include. As an example, I selected the Duet and the Built-In Output and made sure that the Duet was in charge of timing by clicking the radio button next to it. Now in a program such as Ableton Live you can use both the Duet and the Built-In Output at the same time allowing you to cue loops and tracks before you bring them into the main mix.
You could also create other aggregate devices to allow recording on both your Built-In Input and your external device inputs but be warned that the recording quality may vary between the two devices.