I have a Norco Katmandu that has shimano tourney derailleurs and shimano shifters which are low end. I am looking to upgrade some parts to make the bike work better and am wondering where is the best place to start to get the best bang for my buck? I do some road driving and some off road but not enough to justify buying a new bike. I was thinking of getting a new rear derailleur but someone was telling me getting new shifters may be the best place to start. Any input would be appreciated