Are you running your RAM at the default 1.8 volts or have you adjusted the voltage to factory recommended voltage?
Try relaxing the timing to 5-5-5-15 and see if you can get higher.
Oh, wait. I just looked. The nVidia BIOS is a little different with respect to frequencies.
If you are running at 400 MHz X 8, your RAM is running full speed. With a 400 MHz FSB, your RAM is running at DDR2-800. RAM clock is twice the FSB (DDR2, remember). At the same time, at a 400 MHz FSB, your bus clock is 1600 MHz. (quad pumped bus transfers 4 chunks of data into and out of the CPU each FSB cycle).
So if you are trying to make the RAM run at 1600 MHz, you are trying to make it run twice as fast as it was designed to.
If your processor will support it, raise the internal multiplier to "9".