Your tractor is the same as a New Holland TC33D, just a different color. You will find lots of information about this model in the New Holland Owning / Operating section.
Best operating rpm is relative to the task and terrain. For just moving from point A to B, 2000 works well for me. If I'm doing loader work, then it's at least 2500 and for using the PTO @ 540 it is closer to 2700.
The best RPM is the one where the engine isn't lugging and blowing black smoke.