FSB is very important to RAM, its it is a very important factor in your computer overall. the FSB (Font-Side-Bus) it is a link that connects your CPU (Central-Processing-Unit) to the Motherboards Northbridge. There is also something called a Memory Bus, this is a link that connects the system's physicall memory or (RAM) to the northbridge also. Therefore northbridge acts as a hub to which the FSB and memory is connected to. That is how your CPU and RAM communicate. FSB is very important in relation to RAM.
For example, if your RAM is 500mhz but your FSB is 400mhz...the CPU and RAM can't communicate anymore then 400mhz. If your car can go 200mph but you are driving on a road that can go 80mph...you can only go that fast. Often time, the FSB and RAM aren't the same speed so therefore thier speed is in RATIO. The ratio can be 5:4 as an example. The numbers vary. Changing speeds of the ram or fsb can change the ratio which often times can give unexpected results such as the ones you are experiencing.