I don't think it is worth it to get 65 defence because the risk when PKing is a bit high (correct me if I'm wrong) and just stay on 45 defence for Beserker Helm. However, you should not train defence but intstead quest it. The link below should give you an idea on what quests to complete. I reitorate, DON'T TRAIN YOUR DEFENCE.
http://sythe.org/showthread.php?t=1172388