Very newb ballast/lighting question

Question is so stupid I know but, just to be sure...

Currently running a 600w HPS, the ballast I'm using has 600w written on the sticker but I want to use a 400w bulb. do I need a seperate ballast or is it safe to just downgrade the bulb?

which ballast are you using? on my 400 digital ballast I can go down to a 250 in some types of bulbs. on my larger ballasts I have to stick with the 1000watt bulbs.

http://cgi.ebay.co.uk/600W-MAXIBRIGHT-COMPACT-BALLAST-POWER-SUPPLY-/140488714722?pt=UK_HomeGarden_Garden_PlantsSeedsBulbs_JN&hash=item20b5c7a9e2

It's a maxibright compact ballast..

Would it be a better idea to just buy a 400w ballast?

bump? anyone?

It's generally not a good idea to use a different wattage bulb than what your ballast is listed for. IMO, I would just go with a 400w ballast if that is what you are planning on using anyway. It will save you some money too. Good luck.