I haven't found any mention USB graphics cards so hopefully this is new. So I bought a little USB box for $40 off eBay that you just plug into your mac and it adds it as a display! It outputs DVI, HDMI or VGA and works great. So, using it along with my built in MiniDP output, I'm now outputting two videos feeds, using the Advanced Output Setup (unlike the expensive matrox dual/tripleheadtogo solution) off my iMac for $40! You can supposedly add up to 4 of these USB displays so that would be 5 including your MiniDP. I can even run it across the room with a cheap USB cable, and prob even run it off a hub, so you could maybe even run one USB cable across a hall to a hub then out to up to 4 projectors.
The exact model I bought is the "DisplayLink USB to DVI 165". Note, I've only tested using my setup described, but I assume it would work if you added 3 more.
http://www.displaylink.com/support/mac_downloads.php
Hope that helps!