We recently decided to get a mac pro in the office in order to speed up our render times, rendering can be one of our most timely processes and our current MacBook Pro's were no longer getting the job done, causing us to have to leave render jobs run over the weekend. Unfortunately Final Cut Pro 6's underlying code is becoming quite aged and is no optimized to use multiple core macs at all, I expect this to change when FCP 7 is released soon, however when rendering video using compressor 3 we can take advantage of all 8 cores on our Mac pro. The following is how to setup your mac using a virtual cluster in order to take advantage of all the cores your mac has available.

The first step is to setup our virtual cluster by opening up the qmaster preferences in the system preferences pane, in the top option under "share this computer as:" set to "QuickCluster with services" Now move down and check the boxes beside Compressor under "Share" & "Managed". Now is the tricky part, you must select the Compressor service and below you will see a button named "Options for selected services" here is where will setup how many instances this cluster should use, you will probably want to set this for however many cores you have in your machine, 8 cores for us. I have read that having fewer instances, such as half your number of cores, will speed up this process so it may be something you wish to test on your own computer.
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Now that the virtual cluster is setup you can choose this cluster when submitting jobs from compressor. This will significantly speed up your rendering times in compressor by utilizing the full power of your computer. In my own testing I rendered out an hour long HDV video file into H.264 without using my virtual cluster and was only using around 13% of the cpu power available to the machine, when I rendered the same video using the virtual cluster up to 96% of the cpu power of the mac pro was being utilized by compressor to render out my video.