Im still researching projectors but im leaning towards DLP. What kind of connection is the video output on the Imac G5.
That would be VGA. Or technically mini-VGA. You will need an adaptor like this:
If you don't already have one.
$19 from Apple. You then plug a VGA cable into this plus (if you want) a 3.5mm stereo audio cable into the audio output. You can even get VGA cables with the audio cable integrated.
Finally, you will probably want
Screen Spanning Doctor (free, and it's pretty safe...many, many people, including myself, have used it...I use it on an iMac G5 and an iBook G4). The iMac G5 did not support having two different things display on two different screens (Spanning); this program allows for that, since the video card in this computer is perfectly capable of doing it. Then the projector can show something different at a different resolution than what is shown on the main display.