Couple of questions.

So, I have a few questions.

Question #1 How do you keep a 400w Metal Halide Light Cool? I have some computer fans aimed at it and its still not keeping it cool enough, or is it suppose to get HOT?

Question #2 Can I use a 400w HPS bulb in a "Metal Halide" ballast?

Question #3 My plants seem to be very "touchy" at the moment when I'm using 450w CFLs on a 20 on / 4 off schedule with regular waterings.
"Touchy" Meaning - Yellow and what appears to be dying.

I can only respond to your plants leaves turning yellow, tha's usually a sign of N deficiency. I don't think you can run an hps on a mh balast.

Hps bulbs only work on hps ballast and vice versa... Unless you have a switchable ballast (will have a switch to change between hps/mh)

and for your heat issue, they are always that hot. a fan directly on it will help "direct" the heat but you are going to need air exchange. meaning that hot air needs to go out and fresh air needs to come in.

Hmm, ok. Any good home remedies for a N def?

home remedies? im not too sure such a thing exists. its a hobby, not a cold. UNLESS you do some research and find that MAYBE you can brew a tea made with some composted leaves of some sort....when your in a pinch. but that would have to be some sort of wicked pinch to be in otherwise run to walmart or somethin and see what they have to offer. if you happen to be fortunate enough to have a hydro shop nearby go there. alot of people will tell you tho, that those shops are waaay overpriced. local flower shops, nursuries, landscaping companies n shit also carry alot of good shit.

pics would help alot too