Configuring TV output
This is still very much a work in progress. If you have some knowledge to share about getting TV out working, please share it!
If you're using an Analog TV you'll normally be using a yellow AV plug or an S-Video plug to connect to the TV. At this point you should make sure your card even has TV output. If you only have onboard or integrated video you probably don't have TV out. You'll have to buy a separate video card, but don't need to spend a lot of money on one. As often told where it comes to buying components for a mythtv box: buy one with passive cooling.
This information now depends on the type of video card you have.
Nvidia Video cards
Installing the NVidia drivers is a good idea, but not required to get TV-out. You can use a program called nvtv (http://sourceforge.net/projects/nv-tv-out/) to get a great output, with full control of things like overscanning, and it works with the standard open-source drivers! The NVidia drivers won't let you change the overscanning on cards older than a Geforce4, so this is your best option for an old card.
Overscan is how you remove the black borders around your screen. You may find that by using overscan the display is now cut off on the sides. You should try to overscan just enough to remove the black borders without cutting off too much of the display. MythTV allows you to adjust the size of its window to compensate for these factors.
If you want to use the NVidia proprietary drivers, see NVidiaProprietaryDriver.
Note that the Nvidia TV-OUT chips only support a limited number of fixed display modes. Modelines are Ignored. The modes available are dependent upon the TV standard you set in the xorg.conf file. "720x576" is good for PAL.
ATI Video Cards
ATI has begun supplying drivers for recent cards, see AtiProprietaryDriver, which work well with TV-out over composite, s-video, component and HDMI. The ATI driver has superior overscan and picture adjusment for TV-out, it may also produce a better quality image via S-Video or Component. For older cards the GATOS project http://gatos.sourceforge.net may have something that works.
INTEL based Video Cards
Please, refer to the man page http://intellinuxgraphics.org/index.html.
If you're using a Digital TV you should be using a VGA, DVI (HDMI), or Component video connection. You could use an analog connection, but what would be the point of that? :) What you have to do depends on what connectors you have on the back of your TV. VGA is by far the easiest and cheapest, so if you have that things should be pretty easy. Check out http://knoppmythwiki.org/index.php?page=HDTVSetupHowTo for more info, I recommend using Powerstrip to get the correct modeline for your TV.
If you only have Component inputs on your TV, you will need to use a VGA to Component adapter, one of these is: http://www.audioauthority.com/aacconsumers/9a60detc.html If you want to buy a new video card, some now come with Component outputs. Look for 'HDTV out' on the box. I don't know for sure, but it seems that the NVidia GF6200 might be good for this, as you can get one without a fan and HDTV output. These are only just on the market, however, so they may not be good value just yet.
If you have a DVI output from your computer, you might be able to use it for your digital TV. The DVI standard isn't as well enforced as it should be, however, so you might have compatibility problems.
Maybe you want to use the new HDMI connector on your new TV. In that case you may want to try a DVI-HDMI converter from a place such as http://www.ramelectronics.net/html/DVI-hdmi_adapt.html HDMI video is pin-for-pin identical to DVI, the only thing different is the connector type and that it carries audio as well. The same caveat about DVI's incompatibility applies here too.