Didn't someone else mention some modes didn't show up when the monitors had the auto-detected / pnp drivers and were fine one you got those from the manufacturer? (LG or so I think it was mentionned that time)
My monitor in windows just shows up as "Matrox Digital TripleHead2Go". I installed the drivers for my Dell E198WFP, but the new resolutions still didn't show up. I'm working with Matrox support on the issue, but it is very frustrating because they are only open Monday to Friday, 9 to 5 and I'm at work during those times. So they ask me to check something, I go home and check it, then the next day they reply, etc. It's been 3 days or so and I haven't gotten anywhere.
I've found some registry keys that appear to control the monitor resolutions/work spaces for the gxm software, so I'm going to experiment with those and see if I can figure this out myself.