[Myka]

I'm lighting a similar cube... 28x28x18". I have decided on using:

PFO 250w HQI ballast (to overdrive the mogul bulb to produce max PAR per watt)
Lumenarc III reflector (for coverage)
250w XM 10K bulb (for the best output on that particular ballast)

This combination is one of the highest PAR per watt combinations for 250w setups, and will rival an "average" 400w system for PAR (don't believe me? check out Sanjay's Reef Lighting Info pages), but will produce a fairly yellow light, so I'm going to supplement with:

2 - 24" Giesemann True Actinic T5 (possibly 1:1 with a KZ Fiji Pink or Purple)
2 - 24" Giesemann Actinic+ T5 (or possibly 1:1 with a Blue+)

I should get some serious PAR per watt, and awesome color.

I've spent a LOT of time researching lighting, and was really sold on a 400w system, but decided this plan will give me only about 10% less PAR for a lot less electricity!