Could be that your board doesn't like double density RAM if you have any... my old P5B-Deluxe (P965 board) would NOT run Crucial Ballistix memory at 800 MHz. It would only run @ 667 as well. When I pulled the DD stick 800 and higher worked no problem.

Honestly, there isn't a big difference between 667 and 800 in my experience. I did a few comparisons using two different memory dividers in that machine a while ago. Have a look in this thread. The bottom line was 667 vs. 1111 gave a scant increase of 2-3 % for real world applications such as mp3 encoding, x264 video encoding, file archiving, and a photoshop batch run. The theoretical gains as seen by synthetic benchmarks such as winrar's internal benchmark and superpi were greater, but who runs synthetic benchmarks on their machine every day.

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Retired KM user (R4 - R6.04); friend to LH users.