the tv out on rca or s-video on many video cards doesn't work if it is not using the amd or nvidia driver
in your case with that ati x600, you are not using it, this card was abandoned around 2006 by ati
to enable vga and s-video or rca connector you need to use the vesa driver
this driver is alot less than the x1950 and earlier you are using, no 3D acceleration support and graphics will work slow
is it a old tv? or a new hdtv? it is better to use vga connector on those situations
when i wanted to use the dual connection(vga and rca), i used to duaboot windows xp but resolution over rca is a problem, hard to read anything at 800x600
not sure if in your case(with that particular card) you could try a trick, disconnect the vga, connect the rca and turn on the pc, see how far it goes on linux, if not, then dualboot is simpler in this case
remember to disconnect the rca cable when booting into linux if my previous trick doesn't work, some cards tendo to disable both outputs when rca is connected but driver doesn't support it