The type of RAM should be 1024MB-533MHz DDR2-533 PC2-4200. While you can use a high MHz (667) and module (5400), your system will only operate at the 533MHz and 4200 level, so why pay the expense? Also, you should not be mixing the two types. In addition, you stated "DDR" not "DDR2." I think you just mis-entered the data, but please check...it must be DDR2. Actually, you should not be able to fit DDR's in the slots at all.