My area is about the same as yours (2'd x 4'w x 6'h), and I use one 400 watt light (with a digital ballast to minimize heat), and I also use two sets of four 30-watt CFLs at the far ends of the rectangle to provide additional light to the plants more distant from the 'sweet spot' right beneath the HPS light. Two 400s would probably be better in terms of lumens/plant, but that might also be somewhat of a heat problem too, I dunno. In any case, my lighting seems to kick serious ass for that area, and the heat is manageable, even on these 97 degree days. If I were you, I'd try the same thing: start with a 400 and some sidelighting CFLs and see how your temps are. If they're totally fine, zero probs, then it might be worth trying a second 400. (If you can do it, two 400s would be better than one 600, not just because of the extra lumens, but also because you can get a better spread on your lighting, and because you have lighting redundancy in case a light fails.)