I recently upgraded my Mac Pro refurb (from apple online store) 3Ghz quad, 2Gb RAM (4X 512Mb chips) with an extra 4Gb Ram (2X 2Gb from OWC). As I was moving the chips around, I noticed that the apple 512Mb chips were not identical, meaning that: - 2 were produced in the U.S.A. - 1 was produced in Singapore - 1 was produced in Korea, with a different brand name, and the heatsink looked slightly different than the others. If I am correct, each pair must be matched. Doesn't that also mean that they should come from at least the same factory and have the same heatsink construction? I will call Apple on Monday to ask, since it is still under warranty. Any other opinions? Also, I have a memory configure graph that recommends that I locate the chips in this order: Riser A: 2Gb,2Gb, empty, empty Riser B: 512Mb, 512Mb, 512Mb, 512Mb Is that the best way? Thanks.