First off, it depends on the case and it's layout. Second the fans should ideally be the same RPM's. You could get away with one high RPM fan in the rear with low RPM fans in the front(note that I used plural in the front layout) to make sure you have an airflow and not a build up of heat or lack thereof of airflow.
Third, you also need to understand that all fans don't operate/behave the same at different RPM's, most often they don't push as much air as another fan with a different blade design.
That being said, what fans are you working with and how are they connected to your system?