EDIT - Ignore me I was being daft!!!
/facepalm
I have done a little more fishing and the Memory Frequencies on this board allow for the following...
1:1, 5:6, 4:5, 3:4, 2:3, 5:8, 3:5 and 1:2.
To help you understand, those are dividers of your FSB....
400 FSB 1:1, RAM = 400 (x2 for DDR = 800)
400 FSB 5:6, RAM = 480 (x2 for DDR = 960)
400 FSB 4:5, RAM = 500 (x2 for DDR = 1000)
...and so on. Now apparently not all of these dividers are always available. It depends on the FSB.
So you can see that every time you raise the FSB above 400, your memory is going to go with it, even on the lowest divider (which is running in sync) 1:1.
Given that your memory is only rated for 400 (x2 for DDR) then that could be an issue.
So, with that in mind, maybe we should go back to the 9x multi and see where that gets us...