Here's the situation... Mac Pro 8-Core Workstation 3.0GHz, with two sets of 2x2 GB 667 MHz modules (making overall 8GB). There was one set of Transcend TS2GAP685G (2x1GB 667 MHz), dead after one year of working, should be replaced with the Transcend TS2GAP192G (2x1GB 800MHz). The question is will this FB-DIMM 800MHz memory (used for Mac Pro 8-Core Workstation that uses 800 MHz FB-DIMM-s) work on 667 MHz mainboard? I know it should work on 667 MHz, I just wonder did anybody had experiences with 800 MHz modules in older Mac Pro, that is supposed to use slower memory? Incompatibilities or something else?