Supported system bus frequency and memory speed combinations
To use this type of DIMM... The processor's system bus frequency must be...
DDR400 800 MHz
DDR333(Note) 800 or 533 MHz
DDR266 800, 533, or 400 MHz Supported system bus frequency and memory speed combinations
In order for you to use DDR400 you must have a FSB of 800MHz. Since your FSB is 400 it will be reduced to DDR266. Don't forget that DDR stands for Double Data Rate. In other words DDR400 has an effective speed of 800 MHz. Since your FSB is 400MHz DDR400 is actually twice as fast.http://www.intel.com/support/motherboards/desktop/d865gvhz/sb/CS-027109.htm