Ok, so I did a bit homework, the digital ballast is where you can switch from 400w to 600w, and you can use 400w or 600w bulbs(notice: do not try put 400w bulb on a 600w switch, always adjust the same wattage as the bulb, you might burn your home otherwise), soooo you can maximize the control of your plant, for example on the veging days you can put 400w and in flower 600w, and put some heavy buds.
Or if you decide to grow more plants, you can go from 400 to 600w ;o) I will use only 400w but just incase... I will buy 600, whos knows.
I also choose the best ballast
Www.mrhydro.co.uk/lighting/digital-ballasts/lumatek600w.html
Only the ballast cost, 120pounds, but with the cooltube and the hps light combo in ebay you can get for 184pounds. They also offer fan and filter, but the carbon filter I check, they offer pore quality, the best I know is rhino carbon filter, they last longer. Sorry for my english explanations
im lacking a little bit.