Johnny I wonder how you know that, shared memory means its using the internal memory which is slower than the GPU memory usually, at least on the high performance ATI/NVIDIA.
http://www.notebookreview.com/default.asp?newsID=2526
Video performance is not affected by the shared memory - unless there is NO DEDICATED MEMORY what so ever, since Video performance is OVERLAY - direct access to the card's internal memory.
All in all, Video editing performance is govenred by the system's memory and cpu - getting a dual cpu (provided that the video editing software supports it), a faster memory with dual pipelines (which basically means that if you have 2 memory modules with the same speed and size) will be doubled.
For example - 2 sticks of 1GB 400GHz will have the performance of 800Mhz - in one condition - that the bus is 800Mhz too.
There is one exception to this - AMD processors - the new processors have separate interface/controllers for memory and system bus, which makes them utilize the memory performance rather then being limited by the system bus.
If you're really looking for performance, look for P4 overclocking using water or multi peltier modules, with proper overclocking you can get 50% more, but the cooling systems for those things cost more than the whole computer.
There was an article a few years ago about people who took cooling to the max and used liquid nitrogen to cool a cpu to about -150 degrees, they overclocked the system to 10GHz........
http://forums.overclockers.co.nz/showthread.php?s=&threadid=7032
Ofcurse we're talking crazy now, for simple video editing a simple high performance computer (either AMD or Intel) will do the trick.