First of all, is there an actual code on the ballast... like an ANSI rating that starts with an 'M'? This will tell you what type of bulbs it can run.
It sounds like a 400watt HQI ballast, which in the US is really nothing more than a MV ballast (there are no true 400watt HQI ballasts made in the US... they use other similar ballasts).
You can check here...
http://members.fortunecity.com/paulerik/id33.htm
If things get confusing, contact PaulErik (thats his webpage, and he is a regular member here with the lighting info).
IT sounds like you are using the equiv of a 400wattHQI ballast though, and if so, you might want to look into HQI rated bulbs so they last longer... not the typical probe start bulbs you prolly have. The list is short, but mighty...
Ushio/ BLV nepturion (non-CWA models) 10,000K, 14,000K, 20,000K... Aqualine 10,000K, Aquaconnect 14,000K, and all of the Giesemann 400watt Moguls are HQI rated as well. These bulbs are designed to take the higher current of HQI ballasts. But I would look for an actual rating and contact PaulErik here to be sure. Perhaps if you could post a pic of the information plate thats on the ballast?