look boss, if you buy a ballast that is 600w it will run a 600w bulb, it will not run higher, the bulbs are different sizes, 400, 600, and 1000 usually.. all have to have their own, matching ballast, and socket.
im using 1000w at 75% if at all possible, 2 plants... 1000 still not cool enough to run it without blowing up the bill.