I'm also thinking about starting a grow about that size and using a 150w hps. From what I have read (there's a thread somewhere, it was linked from one of the CFL VS. HPS debate threads, but i don't remember where exactly) it is possible to use this size light, but you need to use a cool tube and proper ventilation. Some one had a stealth grow built out of a entertainment centre or something... with 150w HPS for flowering using scrog and fluorescents for the mothers.