Extended Desktop on Optiplex GX620 USFF

My work machine is (as stated above) an Optiplex gx620, Ultra Small Form Factor. I noticed that the graphics card (the one that comes with the machine) is saying it can support dual monitors and gives the option for extended, however there's only the one monitor port on the back of the machine. I did some research, and I keep hearing about a special "dongle" y-cable that can be plugged into the back and the card recognizes it and allows the extended display. However, in all my searching, all I've been coming across are the y-cables that MIRROR the display (what good is that???) so I'm really at a loss.