I like the hydro setup!

Please, please, please, go out and get an HPS light, Go for 50 watts/square foot. Without know the exact measurements of space the plants are going to take up, I would suggest a 400W HPS. You can buy a 400W HPS system for $99 on ebay. Hell, if you have the money, go with a 1000W HPS.

Stay with the flouros for vegatative growth, then switch to the HPS for flowering. Don't forget: Your plants will double in size during flowering, so you will want to start flowering when the plants reach 12" in height, under an HPS setup.

If you are dead-set on staying with flouros for the entire grow, then expect to start flowering when they are even shorter.

Good luck!