I have the exact same memory, a 6400 and a pci-e 8800 gts 320 mb on a 775dual-vsta
Just to let you know what I've run into here (I'm at bios 2.4)
DDR 667 - 333 mhz @ 4.0 - 4 - 4- 12, 1T and pipeline ON: 4842 MB Bandwidth (IS STABLE)
DDR 533 - 266 mhz @ 3.0 - 3 - 3- 12, 1T and pipeline...