To save you some frustration, my experience with bathroom fans has been that they are crappy and over rated.

A 250cfm Stanley Blower is about $50 and will give you enough air movement to cool your lights as well.

Here is a link that might be useful: Stanley Blower

Thanks CR,
Several questions.
Is it as noisy as it Looks?
What about duct work? I was planing on moving the air into and out of the room rather than blowing it around within the room.
Without turning this thread into a big Growing Under lights disscussion, is that much wind necesary to cool a 400 watt H.I.D. in a 120 cubic foot area?
Thanks,
~Ken~

It's not any louder than a bathroom fan; it's quieter than some fans.

A 400 in that small of a space needs a decent-sized blower. You will be surprised at how the heat can build up, especially if the ambient room temp is normal room temperature. You might not need the blower if your space was in something like an unheated garage over winter. And you might not run it full blast all the time - the other settings are I think 50 and 150 cfm.

Where is the exhaust output for your planned ducting? It should be ducted so that it goes completely outside your house, not just into an adjacent room or attic, or else you can end up growing mold, because plants release water into the air.

I was surprised to find out that a 400 watt bulb generates 1364 BTU when lit. In my grow room I vent the air from a single 400 watt HID fixture into an un-heated section of my basement in the winter and it noticeably heats the space. In summer I plan to route the air outdoors because it will make the basement too warm.
In such a small space I would use a fan intended to be run continuously as you don't want it to quit on you and burn down the house. I control my lights with a Hydrofarm Master Digital Timer because it has a setable temperature sensor which will shut your light down if they get too hot, like in the case of a fan failing.

If your light fixture is sealed you can make a ventilation loop running in tandem with the light that pulls air from outside the grow room and vents it outside the grow room. The light will run way cooler this way. a second fan can be set to a thermostat to dial in the temp you want.
Lookup "Jasons indoor guide". His website has "exhaust setup" diagrams.
I've seen plants inches from lights setup this way with no heat damage.

Thanks for the input,
I think I found something that will work. It seems cooling the light will be more of an issue than supplying air for CO2.
I don't want to turn this into a "Growing Under Lights" thread.
Thanks again,
~Ken~