i grow in a attis with the same measurements as you are talking aboutand i use a 1000 watt hps and a heater to keep the temp up at the moment i dont know if you are in a warm climate as i am in ireland so the temps are low here at the moment so heres a link to a light thats getting good things said about it .

http://www.progrow.co.uk/acatalog/fl...t_systems.html