Right now I have a 29gallon but I am purchasing all the equipment to upgrade to a 75 Gallon in May/June right now. If you are looking for lighting the advice I got was to go with a T-5HO setup with 4-8 bulbs depending on the tank dimensions (the more the better) and make sure that they have individual reflectors not a parabolic reflector.
T-5HO bulbs run cooler, are cheaper, and the bulbs last longer than Metal Halide. You can say the same thing in comparison to a Power Compact other than the price although in the long run they are alot cheaper (each bulb is a fraction of the cost of a PC bulb and also better lighting). If you don't mind spending the extra money on the Fixture, Bulbs, and Electricity costs though I would say go Metal Halides.
What size tank so you have right now.