two exteranl displays on an iMac?

I would like to connect two external displays to an iMac I am looking to buy.

I know one is easy and just requires the use of the mini-DisplayPort on the back of the computer.

However, I am not sure how to do the second, I know that there isn't a PCI card slot or anything, has anyone done this with a FireWire or USB external graphics card? If so how well did it work?

thanks!

Monoprice has a good selection based on what you want it to do..

USB to ???

Analog & Digital Format Converters

The new iMacs also have a Thunderbolt port. Later on when (and if) displays support the Thunderbolt technology, several should be able to be attached in daisy chain. At least in theory....

This may help. It converts USB ports into video ports.
Amazon.com: Diamond BVU195 HD USB Display Adapter (DVI and VGA with included DVI to VGA adapter): Electronics

There are several similar devices out there. I'm using a similar device with my MacBook Pro and it works pretty well. Read the details carefully though. Not all of them specifically mention OS X comparability for example. The Device I am using doen't fully support OpenGL. ATPM 15.08 - Review: OWC USB 2.0 Display Adapter