When I got my s775 parts this summer, I only ordered one 2GB kit, but at the same time I ordered the 64bit version of Vista, and later bought same 2GB kit for a total 4GB. RamGuy from houseofhelp.com simply copies this to all "4x1GB sticks threads": "667MHz and 5-5-5-15 timings". May be the case with pre-P35 chipsets...But both my kits work wonderfully together, had up DDR1020 with 2,4x mem divider.
I just changed my OC from 8x425 (2.1vdimm) to 9x400 (2.0vdimm) 4-4-4-11.
One kit reaches higher MHz with 2.0v