Increasing the number of cores isn’t going to make what you’re doing any faster. You’re only using 1 core, with 3 mostly idle. If you upgrade, you’ll be using 1 core with 7 mostly idle. Kind of like saying if I had two cars I could drive to work twice as fast. The only exception to this is if you have your multiple cores configured in a QCluster and are an extremely heavy user of Compressor. Compressor is the only application in the Final Cut suite capable of utilizing more than 1 processor at a time.
Being a heavy Motion user, you might benefit from upgrading your graphics card. And if you’re thrashing your drives you might benefit from combining your internal drives into a RAID configuration to double or triple your effective drive speed.
But the bottom line is, for the current version of Final Cut Pro and Motion, adding cores isn’t going give you ANY speed boost.
Here’s hoping that Steve Jobs’ announcement on the 20th changes that.