Re: XPS 420: RAM Frequency is 667 MHz instead of 800 MHz.
Well mixing the latencies can definitely have an effect, in that the memory controller will try to run all the ram according to the SPD's that are programed into the sticks. Since 6 is slower that 5 the controller will try to run the CL 6 at the CL 5 timings, and most likely there is no CL5 timing programed in the CL 6 sticks, then the memory will fall back to the slowest ram timings in order to prevent the memory controller from self destruction. The most easy way for it to do this is to lower the speed.
You best bet is to have all the ram the same.