reading & Writing to ram should at a 1:1 ratio for best performans if the ram is faster then the CPU BUS not northbirdg's FSB then the ram's READ takes a performance hit if the CPU's BUS speed is faster then the ram it's Write takes a performance hit.
Read \1:2 2:3 3:4 - 1:1 - 2:1 3:2 4:3/ Write
its very simple
if you have a 1333MHz FSB your CPU bus would be about 333.2MHz x 4 plus or minus 1 to 4 MHZ your ram needs to be DDR2 667 LOL should be DDR2 666\2 = 333MHz for a 1:1 R\W ratio
if you have a 1600MHz FSB then your CPU bus is 400MHz x 4 your ram needs to be DDR2 800 or 400MHz x 2 for double data rate for a 1:1 R\W ratio
Your FSB or Front Side Bus is your north bridge Chip AKA Intel P35/\P45 chipset which can hit your rams top speed but that only gives the other hardware like the PCI & PCIe SATA, VIDEO, ect A HI SPEED access to the ram which is already very fast.
So for the best performance your better off matching the rams speed with the CPU's BUS to give the PC's Software a well Balanced Read \ Write to the Catch