Third monitor on a 2008 iMac
You can purchase a USB to DVI adaptor.  There are heaps of them on the market

eg http://www.gefen.com/kvm/dproduct.jsp?prod_id=8863

This will allow you to keep your existing setup and power another monitor from the USB port.

Just make sure that you purchase one that is Mac compatible as a lot of them on th market are not.