If you're in a small space and you're having problems with heat issues with CFLs, I wouldn't use any sort of HID. It's a lot of work for a small gain...you won't get that much more light or flower growth. 8 23-26 watt CFLs will give you around 13000-14000 lumens. Anything over 5000 lumens per sq. ft. will give you good growth. So that's about 2.7 square feet. 18" x 18" is 2.25 square feet. I'd say you either grow 2 medium-good sized plants with each 4 bulb light over a plant, or (maybe) 3-4 smaller plants. Depends on how you arrange the lights, what strain you want to grow, if you're doing LST, etc.