Well i may be totally wrong but when you say your memory is supposed to be running at 400 and its only running at 266 that your comparing the listed DDR speed to the actually MHz the ram is running. So its supposed to be 200 MHz, which is DDR 400, but you have it running at 266MHz, which is actually DDR 533.
I think your getting your numbers mixed up. Cuz if its only running at DDR 266 then thats pretty dang slow. But if its running at 266MHz and DDR 533 then that would make more sence.