It's pretty easy.
As you raise your FSB (CPU Freq) the memory speed rises with it. When your memory reaches its limit, you then lower its divider so its running slower, and push the CPU Freq higher still, with your memory speed rising with it again.
I wouldn't worry too much about overclocking your memory because your are lucky enough to have an AMD system that doesn't need the memory to be overclocked when overclocking the CPU. The 6:5 divider that you are running now is probably the best choice already (your CPU Freq is 240mhz and your memory is at 200mhz (or 400mhz because it's DDR).
You can try to tighten your timings. Or atleast ensure that they are running at their proper settings.