RAM HELP!!

i am stuck...i bought a asus p4p800 delux which supports dual ddr....i guess i want to take advantage of that so should i buy 2 256 ram or 1 512?? If i buy 1 dimm of 512, then i wont be able to use the dual ddr but i might get another dimm of 512 later..... and if i go with the 256 , i might buy a nother 256 whihc means i have 3 dimms of 256.......but i wont be able to take advantage of the dual ddr. HELP!!!!