A 1000 watt really only lights a 4'x4' area. Anything more and your light is too far away to have that type of footprint. Different hoods shade different ways, but 4'x4' is a good bet in general. Therefore, a 5'x6' room could easily fit two 600s or a 1000. The question really is, can you get rid of all that heat? If so...then the more light the better. A 6" inline fan on an aircooled 600 or a 1000 HPS (using the hood to exhaust air from the room) along with a fresh air intake of sorts works well.

However, I too grow in a 4'x6' room, but I run two 1000 watt HPS lights and my room is only 4.5' tall instead of 10. Both lights are aircooled and draw air from the grow room, through the hoods, out 6" ducting, into a 6"-8" Y, and out an 8" 740cfm fan. intake from a different source with a 6" 500cfm fan. run 35 plants per light, in 2 planter boxes that are 4x3 feet each. Each one sits under a 1000 watt.