I'd worry more about exhaust than intake. Nature abhores a vaccuum, so I doubt additional intake is necessary, and it definatelly does not need to be fan-assisted. The fan-forced exhaust leaving the room will provide the suction for the intake.

Check the link in my signature regarding dealing with heat issues. Might be helpful.

Usable light from a 400w HPS extends about 3 feet, max. If you can find a heat-resistant glass or plastic for the hood, you can get the fixture to within 8-12 inches, or so. (more usable lumens to leaf surfaces)