Thanks to all for the suggestions.
Once I determined that it was not faulty RAM or RAM slots, I dove into the RAM tweaking in the BIOS.
Found a post by a guy with the same mobo, RAM, and problem who said that G.Skill told him to change DRAM voltage, command rate, and a couple of other things to try.
FYI, changing RAM speed from 1066 to 800 was not an option. This speed is calculated and read-only (at least in my set up).
In any case, the above tweaks allowed memtest to pass with all 4 sticks and, to date, I have not experienced any BSODs.
And I learned a heck of a lot about tweaking. Thanks again.