My mobile setup has one monitor, and I've never been able to figure out how to have 2, though I have spent some time researching it. Had two external monitors at one time and could not get three full displays. I may be wrong here, but if you use the Matrox splitter or connect on multiple ports (e.g., DVI and HDMI), you're just extending the resolution of the second monitor to the third. Meaning, you're getting the same pixels-per-inch on two monitors as you would on one. This translates to no extra screen real estate -- it's just blowing up one display. As far as I know, laptop video cards only have the capacity to extend their native resolutions to one extra monitor. If I'm misguided, anyone, please let me know.
The people who've recommended the IOgear USB-to-DVI are spot on, great solution. I've had up to 3 externals connected to my laptop using the VGA port and two IOGears. I don't know about playing 3d games on a USB monitor but playing video and normal computer stuff works fine.
FWIW I used a USB-to-VGA and it didn't work at all, switched it out for the DVI version which I've been using for years without major issues.
http://www.elitetrader.com/vb/showthread.php?s=&threadid=217294&highlight=Tritton This Bolimomo info might be of interest .