Intel improved BIOS firmware since version 0400; it jumped from 10J to 10N. Memories at 800MHz require more voltage (1.85V) than 667MHz (1.8V). It is possible to use 1 memory stick at 800MHz, when you have 2 memories the board will fail. You might try to download the voltage. Kingston memory KHX6400D2K2/4G works at 667MHz (1.8V) and 800MHz(1.85V)
Higher memory voltage increases processor temperature and it can damage the system.