UPDATE: Problem Solved: I could go up to a max of 16Gb with 8x2Gb dimms, and a max of OSX 10.7.5 Hi all, I have a Mac Pro running Leopard 10.5.4 (bought on 27th Sept 2007 - MacPro 2,1 - 2 Quad-Core Intel Xeon 5400 Series) with 8 slots avaliable for 667MHz DDR2 ECC FB-DIMM Memory sticks and I want to upgrade to 32Gb RAM. (I'll also be upgrading to the latest OSX as well) I current have 9Gb RAM installed like this: SLOT 1: 2Gb SLOT 2: 2Gb SLOT 3: 2Gb SLOT 4: 2Gb SLOT 5: 512Mb SLOT 6: 512Mb SLOT 7: -empty- SLOT 8: -empty- TOTAL: 9Gb RAM The question is, which of the a) or b) suggestions below would be better for optimal performance, stability and less strain on the Motherboard? (Price is not an issue) a) (2Gb and 8Gb mix) SLOT 1: 2Gb SLOT 2: 2Gb SLOT 3: 2Gb SLOT 4: 2Gb SLOT 5: 8Gb SLOT 6: 8Gb SLOT 7: 8Gb SLOT 8: -empty- TOTAL: 32Gb RAM b) (all slots to have 4Gb each) SLOT 1: 4Gb SLOT 2: 4Gb SLOT 3: 4Gb SLOT 4: 4Gb SLOT 5: 4Gb SLOT 6: 4Gb SLOT 7: 4Gb SLOT 8: 4Gb TOTAL: 32Gb RAM Any suggestions or any other view points welcome. Does it even make a difference? Thanks all.