Usually what you can do is, try connecting your monitor to the other port on the card because on standard ATI cards, the s-video port is shared with one of the DVI/VGA ports. But.. Dell don't use standard parts.
It was hard to find a picture of the XPS 460 fully assembled.
Does your XPS have a TV tuner as well. I think the port you are trying to plug the s-video into is a Video-in on the TV Tuner.
If you are plugging the s-video into the 3650, you will need to install the Catalyst Control Centre. For the TV to be detected, the cards needs to detect 8ohms back from the TV. If it doesn't you can force a detect with CCC - Display Options - Force TV Detect.
Is this your system? Intel Core 2 Quad Q8300 2.5 GHz processor 6 GB DDR3 RAM (8 GB maximum) SATA hard drive PCI Express HD3650 video card Integrated Intel 7.1 channel high definition audio Built-in ATSC/QAM/NTSC TV tuner
cos in this system, the s-video is part of the TV tuner, but the card does have an HDMI port for High Def TV's