So, in the good old days of DDR memory, one simply had to take the fsb of the processor (lets use amd chips), multiply by 2, and you'd get the optimal ddr speed.
Athlon XP with 133mhz fsb = ddr 266
Athlon XP with 166mhz fsb = ddr 333
Athlon XP with 200mhz fsb = ddr 400
... and so...