How important are MHz ratios between the CPU and RAM?

If you have the right type of motherboard the ratios between RAM and FSB are irrelevant. You will probably want to use 800Mhz RAM. But more critical is the latency. Corsair TWINX CMS2 2X1024 C4 is about the best you can reasonably get. It will overclock faster than the 1066Mhz. It has EPP2 compatible and SLi compatible profiles. 667 does not overclock as well as 800 typically does either. There are factors that you need to worry about more than overclocking your RAM at all. First is QVL of the motherboard for RAM. Not all RAM companies create qualified products.