I have bought some compatible 4GB RAM modules, actually 2 x4 dimm packs of different brands, and I'm totally unable to get the machine to boot properly with those installed. I have tried them alone, combined with some of the RAM the machine came with with no success. For information, the machine came with 4x512MB Kingston modules and 4x512MB Hynix module. As soon as I remove then, the machine boots flawlessly. As I'm not very familiar with mac computers, and because first generation Mac Pros seems to be strange beast -i.e. 32 EFI for a 64 bits system-, perhaps I'm missing something. Any clues will be welcome.