fsb:dram ratio is something like multiplier. it works like this:
base clock x dram multiplier = dram clock
let say in this setting:
cpu in stock setting = 2.4GHz, 266MHz base clock,
fsb:dram = 1:1 = 1/1 = 1
base clock x dram multiplier = dram clock
266 x 1 = 266MHz
for this setting, your dram clock is 266MHz.
let say another setting:
cpu in stock setting = 2.4GHz, 266MHz base clock,
fsb:dram = 2:3 = 3/2 = 1.5
base clock x dram multiplier = dram clock
266 x 1.5 = 400MHz
for this setting, your dram clock is 400MHz
let say another setting:
cpu in overclocking setting = 3.0GHz, 333MHz base clock,
fsb:dram = 1:1 = 1/1 = 1
base clock x dram multiplier = dram clock
333 x 1 = 333MHz
for this setting, your dram clock is 333MHz