If there is no gaming involved, you don't need such high-power videocards, because their 3-D processing power won't be used. A low-midrange videocard will give comparable results for your applications.
This
GPU video performance analysis on X-Bit Labs should help you select an appropriate card. In the article conclusion, it is stated that anything upwars of a X1600 Pro is sufficient for video applications.
Please also note that, to the best of my knowledge, video editing, Mpeg2 creation, DVD authoring, and audio editing do not use the videocard processor, as they rely on the CPU for these tasks.
So really, the videocard is only necessary for video decoding.