Firstly, to dual with an iMac or any Mac for that matter, you will need a cable that converts the Mini DVI that the iMac has to the DVI or VGA that your monitor has. I went with a Mini DVI to DVI converter as opposed to Mini DVI to VGA because you get a higher quality picture with DVI (digital picture as opposed to analog). I picked mine up brand new for around $7 shipped on eBay, so if you search, you can find something around that range. Apple charges like $35 for their, so I would suggest looking on eBay or elsewhere.
Secondly, I found a monitor that works. Some monitors say there for PC’s, others say for Macs. Even monitors that say they are for PC’s work fine with Macs. My monitor came with a disk with some drivers and whatnot on it, but I never touched that. I simply hooked up the Mini DVI to my iMac, hooked the other end of the converter to the DVI cord that came with the monitor, and then hooked up the other DVI end into the monitor. I then plugged it in, and turned it on. Then to setup the dual, go to System Preferences>>Displays>>Arrangement, and select what you want. You can mirror them, change the main window, change which side the dock is on, etc. It is quite simple.
So far so good, I really enjoy the extra screen space and the crystal clear picture that the HP w2207 provides. I was able to get it relatively cheap on eBay brand new, but this monitor retails at $450 or so.