400w sounds perfect for that space. I suggest a remote ballast becuase those things get hot and heavy(lol) to hang. Might want to get an exhaust fan also to battle the heat, it doesn't seem like much with a few CFL's now but when you get to that high of wattage(400w HPS) it can build up quickly. And if you do give up, there are many people that would buy the used setup off you, so you can make at least some of your money back.