The problem is that the TV output of video cards like that isn't really up to TV spec and isn't representative of what a television will actually show. This is the reason that SonicFoundry chose to use the firewire output ... you can obtain an actual TV preview identical to what your audience will see (well, at least as identical as you can be considering how much televisions vary) since the signal sent to the TV will be exactly the same as what they'd get from a VCR or a DVD player.

You can use the dualhead Matrox for this purpose by setting the second display to go to the TV out, then dragging the preview window over to the second display, and enlarging it to 100% view. This probably won't fill the screen exactly, and the preview will probably be jittery, you won't see what the safe areas will be, and there's no guarantee that the colors, brightness, and contrast will represent the final output. This is about as close as you can get without using firewire. As a side note, even using this preview method with other video editors will suffer most of these same flaws.