First of all, you cannot change the multiplier on that cpu since it is locked. Unless you have one of the extreme edition cpus, the multiplier is locked.
I'm pretty sure that the multiplier you are using is either 8 or 9 (I can't remember mine of the top of my head), and you have to multiply that number by the fsb to get the total result in mhz. I.E. a multiplier of 8 with a fsb of 350 would equal 2.8ghz (2800mhz). It sounds to me like you upped the fsb on the ram or something...
Basically, to change the fsb with the cpu multiplier, you need to unlock the bios settings, usually by hitting CTRL+F1 or something at the main menu to unlock those settings. alongside upping the fsb on your cpu, you will also need to increase the voltage bit by bit on your cpu otherwise it won't boot.
Basically what I just mentioned is what is described in basic ocing. Just google ocing an e6400 on whatever board, and you're more than likely to get a hit or hits...
I have no idea what that '1200' and '1500' for the fsb is you're mentioning. the only thing I can think of is the total fsb between the ram and the cpu.