Basically, I keep getting HCI memtest errors within 8 hours, even after upping my DRAM voltage to 2.1 (max for this ram) and using the settings recommended by the manufacturer (It's Mushkin 991580 RAM -- 4 2 GB modules with heat spreaders).
I'm considering whether to start messing with the NB voltage, or to try a BIOS flash of a modified BIOS from here:
Thanks for the replies The Mushkin forum support person (Greg, who is awesome, responsive, knowledgeable -- can't say enough) had me test my DIMMs 1 by 1. 2 of them turned out to be bad, I'm going to RMA them.
When I'm running stably I'll post my settings here so that others can use them if they like.