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)
Rusty Trichome Reviewed by Rusty Trichome on . Any advice/potential problems? I am on day 14 of origional potting and having some fun with some good bagseed. But am getting concered that I have no greenthumb. So anything that you might be able to tell me differently I would appreciate it. What is your experience level? (first timer Your Equipment: Rating: 5