SwitchResX is a third-party application for MacOSX that allows for configurations outside of what is normally possible in the Displays area of the System Preferences.
The idea is that while the OS is very good at figuring out what display options are available over VGA connections and what it should do when it detects LCD displays, sometimes media center configurations have displays that need to be tweaked and sometimes you want to go HDMI into a receiver. This thread will gather all info on how to work with SwitchResX and settings used for particular TVs and Receivers.
SwitchResX tutorial on AVS Forums:
DisplayConfigX (not free but an alternative nonetheless):