|The general recommendation for Intel systems (except the P4) is to run the CPU & RAM in sync at 1:1 ratio. And just to be clear, the ratio is in reference to frequency, NOT QDR or DDR numbers. 1066MHz FSB runs at 266MHz frequency, DDR2-533 runs at 266MHz frequency, so that is the ideal pairing.|
To explain a little further, the CPU communicates with the northbridge chipset/memory controller via the FSB, & the northbridge chipset/memory controller communicates with the RAM via the memory bus. Optimal performance is achieved when both buses run at the same frequency. If one bus or the other is run at a different frequency (either higher or lower), it causes the northbridge to compensate for the difference & that introduces latency. And the introduction of latency (aka "wait-states") causes a decrease in performance.
However, there is a point where running the RAM bus faster than the FSB can counteract the latency. I've seen a few statements in forums that the ratio is 1:2, but I haven't found a reputable hardware site to backup that claim. So for a 1:2 ratio with the CPU at 1066MHz FSB (266MHz frequency), the RAM would be run as DDR2-1066 (533MHz frequency).
As for the P4, the netburst architecture was so inefficient that running the RAM bus faster than the FSB helped to mask the inefficiency. The usual recommendation for P4 systems is to run the ratio at 3:4 or 4:5, depending on configuration. For all other Intel systems (P1, P2, P3), 1:1 is the way to go.
Current AMD systems don't use a FSB so none of the above applies. But 1:1 is the way to go for all AMD systems prior to the A64.