Hyperthreading i(1 actual CPU core, but 2 logical CPUs to the OS) s a feature of Intel CPUs only and was present in older Single Core Xeons and P4's. No AMD CPU will support this or something similar. The BIOS setting pertains certain CPU virtualization instruction sets (AMD Pacifica) that ought to enhance performance and operate 64-Bit Guest OSes. Intel has a similar feature (Vanderpool or VT) in their CPUs. Note that you might run into VMotion issues if you have a server where it is enabled and a server that has this setting disabled (I'd advise to enable it on all servers, be it AMD or Intel but you can't VMOtion between AMD and Intel CPUs anyways).
See http://www.vmware.com/pdf/vi3_35/esx_3/r35u2/vi3_35_25_u2_resource_mgmt.pdf for detailed information on CPU virtualization.
I don't know how to check a running host whether ESX has detected or is utilizing the hardware assisted virtualization instruction sets, but I think enabled it in the BIOS should suffice. You can boot the physical box with the CPU Identification Utility from http://www.vmware.com/download/shared_utilities.html and check the presence of the important CPU features. I think the hardware assisted virtualization instruction sets from Intel and AMD are displayed as "Supports 64 bit vmware: yes/no", though I'm not entirely sure about that.