What is the difference between 举行 and 举办?
An answer I found on Baidu Zhidao says this:


  二者的区别在字面上，“行”注重过程，“办”强调事情，进行（集会、比赛等）


The difference between the two words can be seen from the words themselves, “行” focuses on the process of events, while “办” emphases the thing things and the carrying out of such things themselves (meetings, competitions, etc).

The following all use "举行"


～会谈 
～球赛 
展览会在文化宫～。



Hold talks
to hold a sporting event
the exhibit will be held at the cultural palace


The following, here, would all use "举办"


～展览会
～学术讲座
～训练班



To hold an exhibition
The hold an academic speech
To hold a training class




So basically
行 = just doing it
办 = includes setting it up and doing it (edit: or organizing like you said in your original post)