There are basically 7 different video connections used on todays equipment, not all of which are of the same quality. Some digital, some analog. Idealy you'll want to use the best quality connection available for each video device you'll be using in your system. For this reason you cannot assume you'll be using only one type of video connection to the projector. If you are uncertain it might be best to plan for the future and provide a variety of connections for reasons of flexibilty and to allow for any future changes.
Most modern projectors have: Composite, S-video, Component, RGB, DVI, and more recently HDMI video connections. DVI and HDMI are fully digital, though RGB and Component allow enough bandwidth for HD connections.
Component video will have no difficulty with 30-40 feet runs.