It's 7800GT, by the way.
You can think of it like this: 2.0 vs 2.3 GHz is a roughly 15% overall speed increase for a roughly 20% price increase vs. a similarly specced machine. The 7800GT will get you between 50% and 100% graphics speed increase vs a 6600 (I'd assume you'd get at least the full 6600) for the same roughly 20% price increase.
If you did nothing but number crunching (video encoding, say), then the processor speed would be the better investment. But, since a lot of the OS makes use of the graphics card these days and the 7800 is so much faster, it's probably going to be the better investment in most cases. Not quite sure how much either of the apps you're interested in benefit from the GPU, but it's probably going to help out (particularly Aperature--I'm just guessing, but Apple seems to do well at accellerating 2D apps with the GPU; Motion, which is admittedly somewhat different, sees a huge speed boost).
Of course, you can't upgrade the processors later, but you can put in a new GPU, so that's one more thing to think about...