Look closer at the manual. It says something about using double sided dimms.
Basically, if your 256 chips use only one side of the ram dimm then you put both of those in slot 1 and 2 and the double sided chip in the 0 slot
(my terminology may be way off so forgive me)
If they are all double sided then you are out of luck. Atleast, according to the manual I found: http://dlsvr03.asus.com/pub/ASUS/mb/sock478/p4pe-x/e1474_p4pe-x_te.pdf