Bear said:
Actually it's the faster bus speed that doesn't matter. the PC 4200 at CL=4 has a faster clock speed.
The PC 5300 is CL=5. The higher the CL number, the slower that ram (at the same bus speed).
The clock speed that the RAM is rated to handle is PC4200=533 MHz and PC5300 = 667 MHz --- but the Mac will run it at 533 MHz regardless. The term CL stands for CAS (Column Address Strobe) Latency; in brief it is the number of clock ticks that a given bank of memory has to wait to "recharge" before it is ready to do another memory operation.
CAS Latency does have an effect on performance, you are quite correct on that, all else being equal, a higher CAS latency means fewer memory operations per second, because the machine spends more time twiddling its thumbs waiting for the memory. While a higher latency will slow the machine down, there is no evidence at this stage that Apple has engineered Mac motherboards to take advantage of lower latency modules. Apple says essentially nothing about the subject other than they 'support' RAM with a range of latency settings. Now a RAM module that does CL 5 at 667 MHz shoudl probably handle CL4 at the 533 MHz speed that the Mac will run it.
So spending more for low latency RAM is 'probably' not worth it on a Mac. Spending money on higher speed (667 MHz vs. 533 MHz) RAM is definitely not worth it.
Thanks
Trevor
CanadaRAM.com
.