I run a 400W HPS in a 2.5x3x8 closet. It is aircooled on its own intake and exhaust, separate from the chamber's main exhaust. On 12/12 the temps stay in the 70s. Always on, the temps fluctuate from 78 to 84 (but the chamber isn't really meant for vegging just doing it now to start a bunch of plants so I can sex them and get this op up & running. My normal clone/veg cabs are too small for that.)
I use a hydrofarm blower fan to move air through the hood, and an axial fan to pull hot air out of the top of the chamber. When I have mature plants flowering in there, I'll have to pull off the axial fan and switch to another blower, with a scrubber mounted to it.
I'm also working on another cabinet, a really vertically tight space overloaded with CFLs, intended to be a hydro micro-scrog. In there, I don't have room for an enclosed hood, so I mounted all my CFL sockets to a board, that has fans mounted in it, pulling air from around the bulbs into a rectangular "space saver" duct, attached to the top of the board. The space saver duct connects to the adjacent cabinet by going up and over the cabinet wall inside the dropped ceiling, coming down into a 4" duct tee. The tee connects to another vent further down the wall in the chamber, as well as to another blower fan intake. The fan pulls air both out of the grow cabinet at screen level, as well as through the duct attached to the light fixture cooling fans. It blows into a small plastic storage container converted into an air-scrubber, and then out the far side of the
cabinets.
I'm working on getting temps under control, but with a little fine tuning (and maybe not quite as large of a carbon bed in the filter, as the blower is having a little trouble with it) it should be OK for 12/12.
The plant sites in the hyrdo system alternate front to back, and the light sockets are staggered in reverse so the tops can grow up from the screen between the lights. At this point it's just a matter of getting the ventilation right and then doing a test grow to see if I can get acceptable yield with so little vertical space.
For my clone/starter cabinet, I actually cut into an armhole into a crawlspace above and ripped a handful out of the side of a fiberglass AC duct so I could stuff a vinyl duct in there and pull cold air into the cab.
So, to make a long story short, where there's a will there's a way--so long as you're not scared of a million fans and cutting a few holes in walls/cabinets to accommodate ducting.
Hot tip: if you can't afford blower fans, use 4" or larger 12DC or 120VAC cooling fans from Radio Shack. They're about $25 or so. (Of course, they're not nearly as effective but beggars can't be choosers. Buy two.) You can use those soft plastic containers that salsa and other similar things are sold in as couplings to sort of turn them into inline fans, by taking two, cutting the bottoms off of them, and attaching one to each side of the fan. The ducting will them slip over the smaller end of the coupling. Be careful to secure the coupling on the unguarded side of the fan such that it won't come into contact with the fan blades if a little pressure is exerted on the assembly.