Relationship between switching frequency and controller's crossover frequency in buck SMPS
A switch mode power supply is essentially a sampled-data system, therefore the theoretical maximum bandwidth is one half the switching frequency.  Practically the phase and transport lag there make it impossible to close the loop there, so 1/5 to 1/10th the switching frequency is a good rule of thumb.

There are many other considerations in compensating an SMPS- Gain margin, conditional stability, current vs. voltage mode, slope compensation, transient response, etc.  

Check out www.ridleyengineering.com, there are lots of good free tutorials and papers there.