iMac on its side...90 degree rotation
I would suggest not putting the iMac on its side. If you do, you need to add an external fan to force air through the machine keeping it cool. If you don't, heat is now radiating in a way it wasn't designed for, and the machine *will* crash. I learned this myself as i wanted to see what the 27" would do in this scenario. I got about 5 minutes before the machine froze. Turned it back, and no more crashes. So its possible, and you can force the displays to rotate the graphics with a hack, but you have to take care of the heat issue or risk crashes and damaging hardware.
USB video cards get slower and slower on the size of resolution you put through them. I use one at 1920x1200...fine for web pages, email...and small videos. You never want more than say 25% of the pixels actively changing on it or it starts showing lag. A video at fills green and your now down to about 5 FPS redraw on the monitor and even moving your mouse to stop the video is difficult. A USB video card at 1600x1200 or 12801024 would be faster.
So any mini can drive two external USB monitors, but for good performance, you need to do 2x thunderbolt, plus the built in DVI which is only on the high end mini.
I personally do an iMac, display port for DVI monitor, and USB to another DVI monitor for a 3x setup.
Left to right: 1920x1200, 1600x2560, 2560x1440
Having a tall center monitor screws with apps that don't let you choose the monitor of preference. (iTunes, Quicktime, Steam) And forget about "full screen mode" in 10.7, its worthless.