You've to add a few lines to your /etc/X11/XF86Config (or xorg.conf, depending on your distro). Here are a few lines that I added to my XF86Config to get TV-out working.
Code:
Option "NoLogo" "true"
Option "Rotate" "off"
Option "TwinView" "true"
Option "SecondMonitorHorizSync" "30-50"
Option "SecondMonitorVertRefresh" "60"
Option "MetaModes" "1024x768,1024x768;800x600,800x600;640x480,640x480"
Option "TVStandard" "Pal-B"
Option "ConnectedMonitor" "CRT,TV"
Option "TwinViewOrientation" "Clone"
Option "TVOutFormat" "SVIDEO
Those lines were added right after "Section Device". I assume that you have nvidia drivers properly up and running. The generic drivers won't provide TV-out.
Try also searching the forum. That's quite a popular question and it's described at Nvidia's homepage too
Regards!