Im new too (first time growing will happen in about 3 days) .. but from what I've learned during my research...
there are Air Cooled tube light fixtures... which keep your growing tent low temp. than it would be running a regular fixture with the bulb on for 18hours... a lot plants are not humid resistant so i figure they will die or what not..
and also to... 400w HPS Light (giving off 50,000 lumens) will be sufficient for up to 10 plants (Your going to need a ballast for this type of lighting though)
you could use a 400w MH bulb in the weeks of veg stage, then switch to the HPS for the flowing stage. (If you could use both at the same time, this would be best for the plants from start to finish)
I bought a 400w HPS + 400w MH bulb / ballast / light fixture combo on ebay... they are not toooo expensive
if you got any more questions just pm me or something