I'm very interested in reading more feedback on this topic. I still get confused over ISO vs. JIS. Basically, ISO is more of an euro-Italian thing with a short taper, and JIS is most common to JP bike (JIS: Japanese Industrial Standard). You may find the model of your crank on velobase and they may tell what type taper it has. It sounds like you need an english thread 68mm BB. Now determining the length is what I would be interested in hearing others discuss. Typically a 115 or 117 should be OK for a double crank I think.
http://velobase.com/
http://www.sheldonbrown.com/bbtaper.html