None of the Apple desktops come with tv-out. Certain aftermarket cards can be had for around $100 that include DVI, VGA and S-Video ports, but as far as I know, no currently-shipping Macintosh desktop model, nor any Macintosh desktop model sold since 2000 has TV/S-Video out capabilities out-of-the-box.
If you're interested in adding S-Video out to the computer, I would suggest adding an ATi Radeon 9200 Mac Edition card. They can be had for a little over $100, and fit in any 66MHz or 33MHz PCI slot. Note that this is not a replacement for the card currently in the computer, rather, an addition, since the card currently in the computer is an AGP card (not PCI).
In addition to TV-out capabilities, you'll also gain the ability to run two or more monitors by adding the 9200 card.