in some motherboards, raising the fsb increases the ram frequency, as well as the cpu frequency. on some boards, tho, you can unlock the ram from the fsb, and set he ram frequency independently. and on some boards, there is no fsb, but there are many independent busses.
lets just say that you raise the fsb base clock rate from 200mhz to 240mhz, and your stock ram frequency was 800, but, is now 1000, and yuor cpu was 3ghz, but now its 3.6 (20% fsb oc), thats if your fsb is locked to your memory frequency.
with some mobos, it isnt, you can unlock the ram from the fsb, set a ratio of 1:1 or 1:2, or you can just plain set your ram at a particular frequwency, like 800mhz. if your mobo allows for the ram to be unlocked from the fsb, then it doesnt need to affect the ram at all when you raise the fsb, it all depends on your motherboard/cpu.
amd chips, as well as i-series intels's dont even have a fsb anymore, they have a ram bus, as well as other busses, and all can be unlocked from each other. if you have a core2, you can prolly change the ram ratio from 1:1 to 1:2 when increasing the fsb, or it may be locked.
pretty much anything older than a core 2 or athlon 64 will have the fsb locked to the ram.
when you overclock anything, ram included, you usually need to increase voltaqge and cooling.