I used to run 2 x 400watt halides plus about 220 watts of VHO over my old 75g reef tank. Heat was definately an issue, even with two fans in the canopy. I ended up having to run a chiller.

Even if you downgrade and run 250watt halides, you should still run a fan or two in the canopy. If you have a sump, you can also add a fan in the stand pointing at the water surface in the sump to help cool a little bit.