hey dude-
I have a similar grow going in a closet a little bigger than yours (roughly 24" x 38"), I have a 400W HPS like you as well. I am on my second grow. Let me tell you that the first grow was disaster/learning experience to say the least. It is very very hard to grow in such a small space with so much light. This was not something I would have thought of and I am sure you have yet to realize it as well, but 400W HPS puts out an incredible amount of heat. Especially with white walls unless you plan to leave the door to the closet open constantly you will essentially be trying to grow your babies in an oven.

My problem was that the temps. got so high that the soil actually began to get baked by the 400W HPS light. This totally inhibitted root growth and I could not get my girl to grow more than 2 sets of leaves and no more than 3 inches tall. The only way to keep the soil cool enough was to provide so much water that it drowned the plant, leading to mold growth on the stem (damping-off), and overwatering which eventually killed it.

Even with the 2 fans circulating air, you will get to a point where you are only blowing around 95-100 degree air... which doesnt help. Because it is a closet I doubt there would be a way to easily/cost effectively have a ventilation system to bring in outside cooler air and expel the hot air from within. This leaves you (as it left me) with only one option: You are going to need to plan on leaving the door to the closet open 24/7 if you want any chance of a sucessful grow. With your 2 fans (assuming they are strong enough) you may be able to set them up so a fan above the HPS light expels the hot air as it rises and a fan at plant level blows in fresh air across the top of the plant maintaining a lower temp.

Fortunately for me, the door to my closet is in the corner of a room. On the adjoining wall is a large window about 20" away. I realized I could use a large bed sheet to isolate the window and the door to my closet. I am now able to blow in COOL autumn air from the window and have it go directly into my closet, while blowing out the heated air that eventually leaves out the top of the window which i leave open a few inches. If you happen to be lucky enough to have a window near your cloest door I would HIGHLY recomend figuring out a way to use that to get a source of cool air.

If you have any questions or need further clarification/advise let me know on this thread and I'll be more than happy to help you out.

-JPG