I don’t agree with your analysis of cost however, given the choices you’ve outlined, get an i7 processor paired with the best video card.
The i7 is multi-threaded, so 2 cores spawn 4 threads. The video card with max VRAM is required for Apple Motion.
If you can hold off, I’d wait until the intel sandybridge i7 makes it to the iMac. Its just been released for the Macbook Pro and is 4 core vs 2, so 8 threads and better memory management.
The new MacBook Pro with 1GB VRAM and the i7 will outperform the iMac with the exception of RAM based processes, since the Macbooks are still limited to 8 GB.