Not to make it even more confusing for ya but this also depends alot of what type/brand of SE bulbs you are using too. Most German brand SE bulbs (Radium, Ushio, etc...) cannot be fired on the standard probe start magnetic ballast and would require a pulse start (HQI) or electronic to work.
Might as well invest in an electronic ballast IMO so that you don't have to second guess yourself on this. Electronic ballasts can fire any type of bulbs (probe SE, pulse SE, or HQI/DE). When shopping around for a ballast though, pay close attention to its power factor rating, the higher the better ( you want to come as close to 100% as possible)
HTH
D.