Thats because for the most part encoding is done on the CPU. Few rendering programs can use the GPU instead. CS5 is supposed to, I'm not sure of any others. Of those that do, you need to have a CUDA (read Nvidia.) in order to do it.
Check your programs. If none can use the GPU just use any cheap card. Spend the extra money on a faster CPU and/or more ram. If they can use CUDA, get a GTS250 or perhaps a GTX260. Not sure what your budget is here.