Hi there. I have a MacPro3,1 with 16GB of PC2-5300F. Yesterday I received my new memory: 32GB PC2-6400F. For my surprise, the Mac stops at the beginning of the start-up, and the front light keeps flashing in regular intervals. What I know so far: 1. Flashing light implies bad memory 2. The new memory modules I have ar all 4GB 4RX8 PC2-6400F-555-11-ZZ (4GB, DDR2, 800, CL5, ECC). Those modules where pulled from a working IBM computer (P/N 43X5284 ; FRU:41Y2845). 3. No combinations of memory module make it work. I followed the patterns from http://manuals.info.apple.com/MANUALS/0/MA430/en_US/MacPro_Early2008_MemoryDIMM_DIY.pdf 4. EveryMac and AppleSerialNumberInfo, both shows that my mac is a MacPro3,1 (Early 2008) "Eight core 2.8GHz", which should support by spec DDR2 800MHz. 5. This is the module details (1st page, 1st row): http://smartmconfig.com/ext/ind/docs/ds/SG5127FB212851UUDL.pdf Any ideas?