I , too, am running my cabinet with a Matrox G400, using the TV-out via s-video.
I tried running the thing in DOS, but the resolutions supported by this card via the TV out in DOS mode are way too limited in my opinion. Even after running the MAtrox VESA utilities I found on the NET, I still could not get many resolutions to work in the TV out in DOS (all games show up in little box in center of screen, no stretch,etc.). I could get them to look great on the PC monitor, but not so good on the TV.
It is MUCH better in windows, and I have since switched over.
Since it doesn't sound like you're going to be using the dual head tv out feature in DOS, this may not be an issue for you.
So this might just be a general heads up about the Matrox G400 tv out capabilities. They are Great is Win, but limited in Dos.