Problem
I have two Dell PowerEdge 1950 computers with Intel E5405 (quad-core) CPUs. After doing the initial ESX install, I could not install CentOS 4.7 64-bit into a VM; the installer complained that the required instruction set was not present, even though these are 64-bit CPUs.
Solution
You have to go into the BIOS setup screen, select CPU Information, then Virtualization. On mine it was Disabled; changing this to Enabled fixed the problem.