I thought it was pretty much gospel that if you wanted to find out the true DRAM limits, you'd drop the multiplier down a notch or two.
I'm also running the E6600, with Crucial Ballistix DDR2-1000's shown to run at the speeds between 800 and 1000 with a divider other than 1:1.
I'm only cataloging preliminary results at the moment, but running at 9x multiplier, 1520 FSB and 760 DDR appears to be a threshold of instability at latency timings that work fine at lower settings or at different FSB

DR ratios. In fact, my 4,4,4,8 settings are supposed to be good on Crucial Ballistix DDr2-1000 up to about DDR2-850, according to the German review of the Crucials at Technic 3D, and my results are consistent with those results -- provided the system is not running at an FSB

DR of 1:1.
But here's my own gripe, which should be posted in a separate thread.
MEMTEST86+ v.1.70 doesn't recognize the 680i chipset yet. That was "OK" for the time being, but now that I've dropped the multiplier to 8x, with the BIOS correctly posting the 8 x 380 = 3,040 or 3.04 Ghz, MEMTEST86+ still shows the processor speed as 9 x 380 or 3.42 Ghz.
It's been maybe six months or more since the 680i chipset hit the street, and you'd think the developers behind the MEMTEST86+ program would've updated it to (a) recognize the chipset and (b) report the correct processor speed.