If you want to raise your FSB to a higher speed than your RAM supports, you have the option of running your RAM at a lower speed than your FSB. This is done using an FSB:RAM ratio. Basically, the FSB:RAM ratio allows you to select numbers that set up a ratio between your FSB and RAM speeds. So, say you are using the PC-3200 (DDR 400) RAM that I mentioned before which runs at 200MHz. But you want to raise your FSB to 250MHz to overclock your CPU. Obviously, your RAM will not appreciate the raised FSB speed and will most likely cause your system to crash. To solve this, you can set up a 5:4 FSB:RAM ratio. Basically, this ratio will mean that for every 5MHz that your FSB runs at, your RAM will only run at 4MHz.
To make it easier, convert the 5:4 ratio to a 100:80 ratio. So for every 100MHz your FSB runs at, your RAM will only run at 80MHz. Basically, this means that your RAM will only run at 80% of your FSB speed. So with your 250MHz target FSB, running in a 5:4 FSB:RAM ratio, your RAM will be running at 200MHz, which is 80% of 250MHz. This is perfect, since your RAM is rated for 200MHz.
This solution, however, isn't ideal. Running the FSB and RAM with a ratio causes gaps in between the time that the FSB can communicate with the RAM. This causes slowdowns that wouldn't be there if the RAM and the FSB were running at the same speed. If you want the most speed out of your system, using an FSB:RAM ratio wouldn't be the best solution.
AIM/Xfire - pzerofgh