While not directly tied to the AGP speed, the 8x card is likely to be better for video editing if it has features the editing program will use. Cards from both ATI and NVidia commonly include video decoders on the chip now, and some even have encoders in there that can take load off the CPU. It's also becoming more and more common that 3d effects are added to 2d video to modify it.

Core Image is a quick overview of how OS X Tiger will be using the GPU for video effects. A video demonstration can be seen here, at the 1:06:50 mark.

It's also likely the AGP 1x board and card is going to be slower overall due to the older memory and bus subsystems it is likely to have.