Damn, i wish i had your problem! lololol! Thats awesome, 12x24, thats huge! Unless you already have a room for veg, clones, and mothers, i would consider dividing that up. Maybe like a 12x12 flower, 8x12 veg, and a 4x12 for keeping a few mothers and starting clones. in the 12x12 flower youll need about 4-1000w's or 6-600w's, (1000w covers about 36sq ft and a 600 covers about 25sq ft). in the 8x12 veg youll need 2-1000w's with a 600 in between, and i would probably just use 2 8bulb 4ft T5's in the clone mother room. If you already have veg and mom rooms and are just planning a 12x24 flower room, just hang 8 1000w's. I really dont think you want to waste your money on 400w's, you'd need 18-20 of them to cover your floor space.. I also hope you have a few years growing under your belt, this is going to be a big setup and require alot of your time. i would like to also recommend going on youtube and watching tons of vids to see how others are doing it and if you like what they have. maybe search subcool420 or helpful grower or farmerjohn fourtwenty. they all have nice growroom setups and i know on subcool's "the weed nerd" videos he goes out and visits a few other indoor gardens. good luck and keep us up to date as to what you decide to build. i know im looking forward to it. peace