ok, I get you now. The answer is no. Your iMac uses a Rage 128 graphics card from ATI that can not support dual monitors in any way. It simply isn't capable. However, many models do support dual monitors with a fairly simple hack. This includes all Radeon based eMacs and iBooks as well as all new models of the G5 iMac.

Here are some good resources
Information on activating dual screen with Screen Spanning Doctor
Download Screen Spanning Doctor 0.3.2 here
Check compatibiliy here

Sorry you can't do it on yours, but the info listed above will more than likely be new to others who can use it.