It depends on what you are trying to achieve.
If you want more RAM you obviously leave them in. The downside to that is that all the RAM will run at 1333. The stated speeds are the maximum but they can run at slower speeds and the motherboard will select the speed that all the RAM will work at, which will be 1333. But having more RAM can improve system wide performance depending on how much you have now and what OS you are running. Just be aware that a 32-bit system will not be able to recognize more than 4Gb. Going from 2Gb to 4Gb with a 32-bit OS will be noticeable. And going from 4Gb to 8Gb on a 64-bit system will be noticeable also.
If you want faster RAM you take the slower ones out but, honestly, you probably won't notice the difference between 1333 and 1600 if they are the same size.
Edit: BTW your CPU and motherboard also determine how fast the RAM will run. They can also have maximum speed ratings. What you'll end up with is the lower of the 3 speeds, motherboard, CPU, and RAM. So if the motherboard or CPU has a maximum of 1333 you won't gain anything by swapping out the the 1333 for 1600.