Rebuild Gary Fisher Mullet rear derailleur
For explanation of bicycle parts see: Terminology index - a list of bike part names and cycling concepts

The problem you describe may be caused something other than worn sprockets. It may be 'ghost shifting' where the chain hopes back and forth between two cassette sprockets. This may be caused be misalignment or bad adjustment of the derailleur or gear shifters.

I would take your bike to a good local bike shop, have them diagnose the problem and find out what components - if any - are worn out. You may find that you simply need a good tune-up. 

Good news is you can replace the cassette and chain, derailleur etc. individually without having to replace the whole drivetrain (unless the whole drivetrain is worn out of course!)