Valid question... I just hope the plants turn out good...

As to enough HPS with 600W... I don't know. My thinking is; I've heard quote some people say that a 600W bulb is a pretty mid level. So I was thinking of flowering with the 400W MH, then placing the 600W HPS for flowering...
With BGH's system, I was thinking that when flowering comes around, I'll turn ON both bulbs and see if I can deal with the heat. If my fans/cooling system is able to keep up with both bulbs, I'll go for it. Otherwise, I'll stay with only the 600W for flowering...

Of couse, thinking about that right now, I'm inclined to start seeing what kick-ass fan can I get to keep the temp low... hmmm....