I am trying to figure out how to upgrade the memory on my 12 core 2.66 Macpro. The system was configured with 8 GB of RAM, 4 x 2 GB. I want to go to at least 24 GB, but can't figure out whether to go with 4 x 4 GB or 2 x 8 GB, or something else. Issues: 1. Various threads mention that memory should be added 3 DIMMs at a time for optimal (but possibly not real world) performance. Since I have 4 DIMMs now, I'm not sure if this means I'm already taking the minor performance hit so this is not an issue? 2. Since 8 GB DIMMS are available, I was comparing Crucial and macramdirect. I'm more interested in support, guaranteed compatibility and a lifetime warranty rather than a lower price. Crucials 8 GB DIMMs are registered, so they say that you have to remove any non-registered memory. I'm assuming the apple provided memory is non-registered and therefore would have to be removed. Since I have a support contract I will be keeping the original memory so that seems to eliminate the crucial 8 GB DIMMS? When other vendors offer the 8 GBs, will they all be registered? macramdirect which offers only up to 4 GB DIMMS for now says "MacPro firmware prefers not mixing 4 GB with other modules". Their price for Apple Factory original 2 x 4 GB is $400, or $800 for 16 GB. I'm leaning in this direction since my configuration that way would be 100% apple. Any advice would be appreciated.