Video out
If a computer is used to generate a video image, this can be done in several ways:
HDMI
If the computer has an HDMI output and the screen / projector has an HDMI input, this is the best solution. HDMI is completely digital and has a high bandwidth.
Composite output
A video card such as a Matrox Parhelia APVe has a good composite output. Matrox has one of the best composite outputs available on a standard VGA card. If you then choose the feature display mode, the image that comes on a hardware overlay will be transferred to the composite output.
An Nvidia card (Asus Gforce 7600GS) is also a good choice.
Distance
Video cables have a short maximum length (analog cables shorter than digital ones). HDMI works well up to 15 meters, but if really huge distances have to be transferred (for example 25 or 100 meters) then it is advisable to have the signal run digitally over a fiber optic cable.