It all depends on the motherboards compatiability with which RAM is better suited to your needs..Say you Have a board that handles 2100 MHz RAM well if your OCing this extra RAM speed stability allows for much higher OC's with stability...
Example:
Stock Speeds
RAM 2100 MHz
CPU 3.2 GHz
CPU Multiplier x16
FSB 200 MHz
OC ed
RAM 1600 MHz OC 2000 MHz
CPU 4.0 GHz
CPU Multi. : x16
FSB: 250 MHz
This is where the extra RAM speed stability is required under an OC scenario...You see by setting the RAM which is rated to 2100 MHz to 1600 MHz in BIOS then setting the FSB clock to 250 your RAM is then at 2000 MHz..Which is a 25% OC...
@akxpckwb---> Where did you get that math from? 800 MHz FSB get real, that would equal on the above setup with a CPU multi. of x16 would be 12.8 GHz...You need to learn your BIOS and computer a little better.