if you double your speed (800MHz to1600MHz) but half your CL (CL 6 to CL 12 [half speed is a number twice as big]) your RAM is going to retrieve twice information at half the speed.
The advantage to a higher MHz is that as your MHz goes up, you also get more bandwidth, but if your CL is higher also, your ram will not be faster.
Since they are both cas 9, the 1866 will be faster, but since that speed isn't even supported... your PC might not even boot at that OC. But anyway, ideally you would get 1600 MHz ram with a tight timing. 8-8-8-24 would be great.