iMac 17" 2nd monitor question
Unless you bought a very old iMac, its external display port is most likely mini-DVI. If you have something as old as a G4 (the "desk lamp"-looking iMacs), it may be a mini-VGA port. A helpful resource for identifying your model and seeing details of its capabilities is everymac.com - those are the only options I see for any of the 17-inch iMac models.