Lol thats a good point Mobay.
But BlueDragon, some ballasts can accomodate either a MH or HPS up to whatever wattage it is for.
Although I think he said that he already has a MH bulb (meaning he has a MH ballast aswell), and also a HPS ballast because he said 'bulb and all'.

But do you have a seperate ballast for each bulb?
Oh and by the way, you can use a HPS/MH (whichever takes your fancy) for vegging and flowering - you just wont get the same results as you would using two seperate bulbs. Though there is someone I know who grows occasionally, and he uses a 1000w HPS and a 400w HPS together and last time he grew (Christmas... I got many happy things that day, namely an ounce of organic) he got a good few pounds. Ill try and find out how many plants he grew and what strain. I know it was a sativa, I just dont know the specifics.
Coulda been White Widow.