They control the speed that your memory runs at. Setting the 1:1 ratio would allow you to run your ram at the same speed as the fsb. So, having the fsb at 133 would have your ram running at 133. But, let's say your system, for whatever reason, cannot run at 133fsb. Running your comp at 100 fsb would not use PC2100 DDR it's potential (only using it as PC1600=100*8*2 although I can't remember why it's that way). So if you put in a 3:4 ratio, your fsb would be at 100MHz, but your ram would be at 133 or PC2100 speeds, due to it taking 100 and multiplying it by 4/3 (although this might not be the case on all motherboards).
