More and more apps are and will take advantage of more cores. Compressor can do virtual clusters (you need at lest 2GB/core of RAM. Programs like Magic Bullet Grinder (processes Canon clips) can encode one clip per core at the same time so a 12 core will process 12 clips in the same time as 8-core will 8 clips.
Eventually FCP, Motion and Color will take advantage of multiple cores (hopefully summer 2011).
So if you have the money the system stays stronger for a little longer.
But all that said the new 8-core will be able to do anything you throw at it. So there is not a right or wrong choice here. But you need to look at a system and not just pieces. I would not spend the extra $1500 and not get a good video monitoring card and monitor.
My 2 cents.