I do have a server with this processor:
It does support enhanced speedstep but I it's not available in ESX4i ?
Also the processor is detected as a socket 478 instead, but it's a socket 775 processor.
ESX4i runs fine, 32bit OS'es ofcourse but no error's
SpeedStep needs to be configured and enabled in the BIOS - which may or may not be possible - and then enabled in ESXi through the advanced configuration option "Power.CpuPolicy" - set this from the default "manual" to "dynamic".
Please award points to any useful answer.