we need more information, such as the model of your mp and what version of osx you use.
I assume you have read the big sticky post about graphics cards in mac pros, yes?
The Matrox card appears to be a H.264 encoder:
This is a totally different beast to the GTX 670. I'm not an expert, but I would imagine that you'd benefit more from the 670 or a similar card, and if you really needed fast H.264 encoding you could probably run the Matrox card as well. As far as I can tell, the Matrox card is not a full graphics card, just an encoder.
670 should speed up encoding too. Not in direct way, because it isn't GPU accelerated in Premiere. Card will offload MPE functions from CPUs making them fully available just for encoding. If you do a lot of Bluray or /and other H.264 files, Matrox card would be a good companion for 670.