A double-bright at about 18 inches keeps me comfortably in the low-light range for my non-co2, low-tech set-up. I agree that you may need to double-up on a cube, and that research into a finnex fixture is probably worthwhile.
If you have the money to invest in one, I would recommend purchasing a PAR sensor for a multi-meter. Best piece of equipment I have purchased since keeping planted tanks, and you won't be left guessing on light intensity and coverage. I was running CFL fixtures before purchasing one, and was surprised to find the CFLs were putting me into the high-light range. I think the PAR readings are critical for any DIY build using CFLs.