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My advice, if you can afford it, buy lots more ventilation than you think you will need!
When I did my setup, I wanted "extra" ventilation, so I calculated what I thought was 2x more than I needed.
:S2:
Lets just say I wish I had more...
If your profile is correct, you're a southern grower like me. Heat is going to be your biggest challenge, if you try to grow inside in the summer.
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)