An electric ballast shouldn't take up any more space than an HQI, but it will be more energy efficient. But what $$$ are you going to save by switching ballasts? Probably not much.
The most energy efficient thing I've seen recently is if you buy 175w ballasts and run the 175w Ushio or Iwasaki 14 or 15k, I forget which of which, but whatever it was, it put out as much PAR as most 250w bulbs. Granted, the bulb costs $100, and you would need new ballasts, but it'll save money I suppose.
I've been trying to get a firm answer from people about saving energy by switching to T5, and the answer I keep getting is that a watt is a watt, 8x54w for 48" tubes is going to be 424w, or 2x250w will be 500w, so you could go with 4x54w for a big savings, but it depends on your tank depth, depends on what corals you want to keep, etc.
I would go as far to say that as long as the tank is shallower than 24", you would be good to go with T5s with single reflectors (IceCap SLRs or Tek). I would say 8 bulbs for a 24" wide tank, 12-18" you could get away with 4 bulbs. It's all about you and your corals though.
