First of all, what do you mean by "I use a CPU with 1600Mhz" ?!
Most CPU's can support RAM speeds @1333MHz these days. AMD FX chips support upto 1866MHz and Intel IB chip support RAM upto 1600MHz.
Frankly speaking, except for AMD's APU's, RAM speed doesn't affect performance by any significant margin for most tasks. However, what is more important is Memory timings.
A 1333MHz rated RAM with CAS timings of #7 is definately better than 1300MHz RAM with a CAS timing of #10. That's the reason why such chips are a bit costlier too.