A 600W HPS isn't going to produce much more heat than a 400W. But I also agree that if you're really wanting to fill an entire 6x5 room with flowering females, a 1000W system would do this nicely. Upgrading your current ventilation system doesn't take much time, as it basically consists of installing larger fans in place of the smaller ones. But, if you're hoping to have an area this large filled with flowering females anyway, then a good ventilation / odor reduction system should have already been in place anyway.

Also, if you can afford it, digital systems are reported to cut down on heat (both the ballast and bulb) by 15%. This is certainly something you'd want to think about when deciding whether or not to switch to a 1000W system. Digital ballasts also save energy during start-up, and supposedly use less overall power during run-time. Just something to consider.