You need to read the link in this forum, about the P5B and the boot straps, seems like this is your problem. Your multiplier for your cpu is what determines the speed of your NB stap....very odd....any how, by lowering your multi on the cpu, it ups the OC on the NB. Use SiSandra 07, and the mem bandwidth test. It will show you the effective speed of your NB, this has been affecting my OC as well. When I set my FSB to 460 and use a 7x, I get 3.22ghz cpu and NB speed of 657mhz. Attachment 35647
Now if I just lower the multi 1 notch, and run the test again, the NB is at 754mhz! *Also, I leave all mobo related voltages @ auto now, as I have had better results clocking above 400mhz that way.
*Keep in mind the NB strap, resets itself around a 50-60mhz spread, so try to set your OC for the 'top end' of one of those fields for best RAM through put speeds. Heres a copy to link.