I have noticed with the same machine (except with a 9800) that even using The new Edge blending and widescreen overlap features that the graphics card is the bottle neck.
I have even had one card the overheats and I need to remove the card directly above it to keep it working.
After that experience I don't think I would even try the Matrox boxes on a machine/card that old.
The cards were great in their day but that day has passed.
Cheers
Toby