As I understand it, the way they have the RAM set up accounts for this. The RAM is installed in pairs. Each half of the RAM bank operates seperatly, so if you have a 256 MB chip at 400 cycles per second that can read or write twice per cycle in slot 1 and the same in slot 5 (not sure exactly what the arangment is) the machine sees it as a block of 512 MB RAM that can be written to FOUR times per cycle at 400 Mhz. If this is so, this RAM will match system buss speeds up to 1.6 Ghz.
I might be wrong about this, but that's how I interpretted various documents. Anyone know any better?