Just an S-video cable, or S-video to composite converter and composite cable if your TV doesn't support S-video. Then just do Detect Displays in the Displays section of System Preferences and don't use mirroring.
That surprises me. In that case, you're pretty much screwed, unless you go out and buy a VGA to S-video/composite converter box. It'd probably be more cost effective to just get a newer video card.