Depending on how the dimms are, newer dimms run at 3.3v and older ones run at 5, there are some PC66 dimms that run at 5v most dimms from the last 3-4 years or so should be 3.3v as to the speed, the faster dimms should work in slower mobos but not vice versa you may want to order the dimms on the mobo so that the fastest and largest go into the first slot, and if at all possible for best performance try and match manufacturor, size and speed -munir