For certain tasks there apparently is a significant difference. The peak transfer rate of 1600 is double that of 800MHz.
However as a normal user you would most likely not notice a difference. If you are planning on purchasing, 800MHz DDR3 is not sold very much, 1066 and 1333 are what you see the most. And all macs that use DDR3 are sold with or used to be sold with 1066 or 1333.
BTW, do you maybe mean DDR2 800MHz and DDR3 1600MHz? because the difference there is bigger.
If you look at a performance increase for an integrated graphics card then it is different. On this forum several have put 1866MHz RAM in to their 2011 minis (w/ Intel HD 3000) and reported a decent increase in graphics performance.
So if you see a difference or not from 800 to 1600 depends on the usage of your computer.
