iirc Asus only support 1 Dimm at ddr400 speed on the mobo,
but it does support some dimms rated for ddr400
running at ddr333 -pc2700 speed.
Fwiw I have 2 256 meg dimms running at ddr350 on a7v8x-x
one is pc2700 and one is pc3200. They run together stable
at cl2 and 1t command rate (minimums)
-That would be harder to do if the dimms were 512 meg,
but I think you have some room to experiment with running
two pc3200 dimms on the mobo -if you are unlucky just run them
at ddr333 speeds :/