My 120g is a in wall setup. Heat will definitely be an issue, unless you can find some way to vent the room. Any kind of heat that is generated by your lights will start to build up. Look into adding an in-wall exhaust fan and a return air grill so you can pump fresh air from the house into the fish room and vent the heated air in the fishroom back into the house.

Here's an example of the fan I'm talking about
http://www.amazon.com/509s-8-inch-th.../dp/b0002ytnb6