Are there relation between CPU's Ghz and RAM's Mhz?

Question

Could I predict what frequency of RAM should be knowing frequency of CPU ?

Answer 1

Today not really. In modern motherboard designs the RAM and the CPU are not that tightly coupled as far as the clocks are concerned.

In the past in many cases the CPU base-clock and the RAM base-clock had to be the same or have a fixed ratio.
E.g Socket 370 Pentium designs had a 100MHz base clock and the corresponding 370 Celerons used a 66 MHZ clock (both directed from a 33MHz system clock that also drove the PCI bus and the RAM). The matching RAM could either be 66 or 100. As you see from that they all used a 2x or 3x clock-multiplier.