Lets use the 6600GT for a reference calculation
Key components of the calculation, Effective Memory Speed, and Memory bit.
1000Mhz <- Effective Memory Speed.
128(bits)<- Memory bit * 1,000,000,000(Hz) = 128,000,000,000 bits/sec
(128,000,000,000/8) / (1000*1000) = 16,00Mb/sec 16.00GB/sec
So, when overclocking you know what kinda bandwidth your getting with different mem speeds, this same method can be applied to system Memory

Key components of the calculation, Effective Memory Speed, and Memory bit.
1000Mhz <- Effective Memory Speed.
128(bits)<- Memory bit * 1,000,000,000(Hz) = 128,000,000,000 bits/sec
(128,000,000,000/8) / (1000*1000) = 16,00Mb/sec 16.00GB/sec
So, when overclocking you know what kinda bandwidth your getting with different mem speeds, this same method can be applied to system Memory
