What's the relationship between Nanoseconds and the Mhz Rating with Memory?

Nanoseconds and Megahertz are inversely proportional.

1 second = 1,000,000,000 Nanoseconds
1 Megahertz = 1,000,000 Hz
Hz = 1 / x seconds

Finding the correlation between the ns rating and the mhz rating is useful as the ns rating can help establish a rough upper boundary especially when overclocking. This is especially relevant with video memory- manufacturers sometimes use memory that is specced higher than what is called for on a particular card.

An example would be the 9600XT. Default memory on that is 300 Mhz (600 DDR). That translates into roughly 3.3 ns memory chips. Some cards have 2.8 ns memory; so what this means is that we can find out factory max spec for the memory. 1 /(2.8 / 1,000,000,000) =~ 357,000,000 Hz =~ 350Mhz. The 0.4 ns translates into roughly 50 Mhz extra that we can likely pump out of the memory. For a person just starting to dabble in overclocking, it is a pretty safe bet that it will be ok to run the memory at 350 Mhz (700 DDR) without a lot of problems.