my 33 was 36x12x18 and I had a 175mh on it without too much trouble..it shed a similar (visable) amount of light that my 192w power compacts did, but had the super cool shimmer to the water
however, I would recommend either a sump or a fan in the canopy blowing on the bulb.
also, in a similar scenario as steve, I've had quite the assortment of light, but I found it neat to learn for myself how different lights work for tanks and what benifits/drawbacks each kind has...I am however working my way towards 2x250w hqi's. too bad they're $$. but the plan is to never have to upgrade after that.
I found power compacts (for me) had the most noticable drop in output over the shortest amount of time, and they're not overly cheap to replace every 6 (or less) months...actinics would only last me about 4 months at 12hrs a day