I have a hard time finding info on exactly which combinations of memory are doing exactly what on the Mac Pro 8-core. As far as I understand, full memory speed is only achieved with four memory modules, since the machine can access four in parallel. I did get four 2 Gb modules (800 MHz) from OWC and I’m very happy with them, but there are the two 1 Gb modules (also 800 MHz) that were delivered with the machine and what do I do with those? If I plug them in on the top memory board as the manual advises, will accesses to these two modules be slower than accesses to the rest of memory, or will all memory access slow down?
To find out, I did a very unscientific test. I plugged in the two 1 Gb modules, so I had a total of 10 Gb of RAM in the machine, then booted it up. Ran a few programs and then started WoW. Had my character run around in a circle in Gadgetzan and saw a consistent jerkiness. The framerate according to Wow was 29 fps.
Powered down, pulled the two 1 Gb modules, booted up again and went back into WoW. Again ran around in a circle in Gadgetzan and had 60-65 fps all the time. Gone was the jerkiness. Conclusion: it’s not worth it to plug in pairs of RAM you’ve got lying around. Stick to foursomes.
Oh, BTW, if your machine becomes sleepchallenged (reboots when you try to wake it from sleep) after swapping memory, do the pull-all-cables-wait-15-seconds-then-plug-in-again, plus the parameter RAM reset (cmd-opt-P-R) one or more times. At least, that fixed it for me.