I believe these to be accurate, but dont bet your life on them.
1ns = .000,000,001 seconds
7ns will run 142.8 mhz (285 DDR)
6ns will run 166.6 mhz (333 DDR)
5.5ns will run 181.8 mhz (362 DDR)
5ns will run 200.0 mhz (400 DDR)
4ns will run 250.0 mhz (500 DDR)
3ns will run 333.3 mhz (666 DDR)