Also take a look at USB-to-DVI/VGA converters. There are many different manufacturers, but I heard that almost all are made by
DisplayLink.
I recently purchased
this one on ebay for $65. I currently have a 24" 1920x1200 screen using mini-dvi and a 20" 1680x1050 screen using the USB-to-DVI converter. It works flawlessly! Moving windows goes smoothly. You CAN notice it's over USB when you focus on it. Scrolling of pages goes extremely smoothly too. Do note that it's not intended to playback video or play games! (although I was able to play a youtube video)
I think that if you don't mention it, someone who's not familiar with your setup won't notice the screen is driven over USB. I highly recommend it!
It has a lot of advantages compared to the Matrox DualHead2Go too. It's cheaper, expandable (up to 4 screens over USB on OS X, 6 on Windows), supports different screens with different resolutions,...
Advantages of the Matrox DualHead2Go include that it supports up till 1920x1200 resolutions (DisplayLink supports up till 1680x1050) and probably can play movies/games.
Note: If you decide to buy it, you have to download the OS X drivers from
this DisplayLink page.