It really depends on what you want to keep. If you only want soft corals, the T5's are getting popular. I was considering them myself. If you want some LPS corals, you might want a couple 175 MH, at least. 250s would be better. If you want clams, like the maxima, then you will need at least 250s.
I plan on using two 250 watt 10k mh and two T5 actinics on my 75 with softies, LPS, and clams.