If you have an HDTV with HDMI, DVI, or even VGA input, then you just need the appropriate video adapter for the output on your mac.
Then setup the TV as your main monitor in the System Preferences>>Display under the Arrangement Tab (drag the little bar at the top of the monitor to the top of the other monitor to switch the primary back and forth).
Now startup FrontRow and / or iTunes to watch your media. If you want the audio through the TV, or a sound system, you'll need to connect the Speaker, Headphone, or Line-out to the TV or Audio system as well.