Which approach category minimums should I use when conducting an instrument approach?
According to the FAA Safety Alert for Operators issued on 09/20/2012 (SAFO 12005):


A pilot may never use the approach minimums specified for an approach category lower than their certified approach category, even if the actual approach is flown at a speed that would be in the lower approach category.
A pilot must always use the approach minimums specified for an approach category higher than their certified approach category if the actual approach will be flown at a speed that would be in the higher approach category. 


Note that this is different in other areas of the world (Canada for example uses the actual approach speed of the aircraft to determine the approach category).