I have a Dell PowerEdge 2600 server with 512MB of RAM. I wanted to upgrade the system to 2 GB of RAM so I purchased 2-1GB PC3200 ECC Registered sticks of RAM. When I replaced the existing 256M sticks the system would beep when it ws powered up and would not display any POST information. I went back to the original configuration and it booted up with no problems. I then put the new sticks in the next two slots and the system booted up, but complained about mismatched or unsupported memory and disabled those two slots.
I called Dell tech support and they told me that the PC3200 was too fast for the system and I needed to purchase PC2100. I always thought there was no harm going to a faster bus speed, but you could not go to a slower bus speed. Are they kidding me, or what?