2 external monitors on iMac

Howdy,
I have a 3.06 C2D 27" imac w/ 4gb DDR3 ram and 256mb ATI HD4670.
Currently I'm running a secondary monitor (22" widescreen westinghouse that I adjusted the original stand to use as a Portrait vertical monitor). I have a few extra monitors lying around and would like to throw a 3rd one on for fun...but so far the only option i've found is something along these lines...
DIAMOND BVU195 USB Display Adapter (DVI and VGA with included DVI to VGA adapter)|GearByDiamond

Anybody use one of these and if so, how are they? Also, am I missing something or is this my only option IF I really want a 3rd monitor.

Thank you much

I am using an external USB monitor driver similar to the one you posted. It works just fine and I have a 23" widescreen via DVI connected. It was a little tricky as SLeopard did not pick up the box on connection probably as the device is not current generation. Once I located and installed the software no issues other than occasionally it fails to "see" the external. Unplugging and plugging the USB re-establishing the connection makes it all fine. The third monitor works for Window 7 under Parallels also.

One thing I did note is that I have to use SMC to increase the fans as the video card was getting very hot and causing freezing of the system when using 3 screens - I have another 23 inch connected by miniDisplay port also. This is the only option I found for adding more than 2 monitors.

Monoprice has a few options as well.

Analog & Digital Format Converters