Replaced the logic board in a 2GHz, Two Processor, 8 mem. slot G5 Tower.
Boots fine. ASD tests all pass. Thermal Calibration was with one error but then successful. (Reported a MLB speed of 800Mhz and CPU speed of 2GHz. Told me to replace CPU with a slower one. Ignored and calibrated okay.)
Machine (and fans) seem to run normal. Temps are good. Problem: System Profiler only shows four memory slots! I can even put only one pair of RAM in the slots that don't show up and the machine runs fine. Profiler shows RAM slots as empty! I'm very puzzled.