Bob, The 1900 does not support using 2 different processors. Also with the 1900 it is dependent  on the motherboard version what specific processors you can use.
I'm using a PowerEdge 1900 server, I'd like to know if I can install two different CPUs, and how different can they be ? Is same family (eg. santa rosa), different clock allowed ? Can I mix dual core and quad core CPUs ? How will the system behave in such cases ?
Can I mix CPU types in PowerEdge 1900?