If you wanna just completely replace your monitor. You just need to plug in the projector to the VGA or DVI port on your video card. You'll need a projector that can handle HDTV if you want to go higher than 800x600 resolution.
If you wanna keep your monitor and have the projector too. Just get a video card with dual video outs. Plug monitor into one, and the projector into the other.