The GeForce3 with a have or both a VGA or DVI output, but not ADC which is what the 20" Cinema display uses.
If it has a DVI connection on it, then pick up the Formac (
www.formac.com) ADConnector for $79. That is all you need. By the way, I just bought one of these from
www.ecost.com and paid only $78 total, which included overnight shipping.