Stuart - You are correct. You must enable the Virtualization option in the BIOS in order to be able to run 64-bit VMs with ESX - or any other virtualization. With 32-bit VMs, ESX does everything in software. THe 64-bit VMs rely on the hardware virtualizaiton capability in the processor to do some of the work.

This wikipedia article is a good starting point - although it doesn't really say how the hardware virtualization in the CPUs work - http://en.wikipedia.org/wiki/X86_virtualization

Todd

Thanks,
Todd