Since sometime in late 2008, Apple's 17" and select 15" Macbook Pro models have included not one but two graphics processors, the nVidia 9400M and 9600M GT. It is my understanding that by default the laptop uses the integrated lower-wattage 9400M, which includes 256MB RAM and helps extend battery life. It will run the main laptop display and push an external display as well. The additional nVidia 9600M GT graphics option is actually a discrete internal PCI-E card featuring 512MB of RAM and offers significantly faster Motion GPU rendering performance. You can see both of these cards listed separately in your System Profiler. So how do you tell which card you are using, and how do you switch between the two cards? I'm glad you asked, grasshopper. Read on and I'll tell you how...
First off, how much of a performance difference is there between the two graphics cards? Quite a bit, actually. Mac benchmarking gods Barefeats have benchmarked the 9400M and 9600M GT in an Apple Motion GPU rendering task (using one of the included HD templates), and saw approx 2.5x to 3.5x faster render times using the 9600M option. That is quite a difference. I was not able to find any benchmark numbers for Compressor or FCP-specific tasks, but we know that FCP leans on the GPU for RT performance. And Barefeats has also benchmarked the two cards for 3D gaming. Again, the 9600 spanked the 9400 by a wide margin. So it stands to reason that you'd see performance gains across the board with the 9600M when using Apple's Pro Apps (at the expense of battery life, of course. Also, your lappy may run a little hotter.).
Now that we've established that the 9600M GT option is A Good Thing for those of us working in post-production, it's time to check which card is currently in use. Open up your System Profiler. This can be found by clicking the apple in the top left of your screen > About this Mac > More Info > Graphics/Displays. This will show both your graphics cards (if so equipped). Clicking on a graphics card in the list will show the displays currently active on the card. You should see the Macbook's "Color LCD" and any connected external displays listed in the panel below. For the purposes of this tutorial, we'll assume that yours shows the 9400M card connected to your display(s).
Switching between graphics cards is a simple process, but annoyingly you must logout for the changes to take effect. So first you should save and close any current work. Now go to your System Preferences and click on Energy Saver.
At the top of the Energy Saver window you'll see two toggles for Graphics: Better Battery Life and Higher Performance. You can mentally read these options as 9400M and 9600M GT, respectively. Tick the Higher Performance option to enable the 9600M GT, and you'll be prompted to save your work and logout.
That's it. Once your Macbook logs you back in you can confirm the graphics card switch in your System Profiler.
Enjoy your newfound GPU processing power!