Quijinn, a 200/50 is actually a little taller than a 180/55, (1mm) so I guess it's splitting hairs as far as tire size.
On another note, this whole post concerns me. Buboo88, if you don't even know what different sprockets do, why are YOU trying to attempt to do this job? You can ask all the questions you want and get answers but that doesn't mean you're qualified to do the work. The chance of screwing something up can be a lot costlier than taking it to a dealer. Better yet, find someone you know that knows about this stuff, watch how its done, then attempt it yourself the next time. Years ago, when I was a wrench at a dealership, we had a shelf in the service area that had a sign under it that read "DO IT YOURSELF AND SAVE". You should have seen the shit on that shelf!
Scott