If you are running a HA machine and want to make sure that your hardware will support 64-bits guest operating system you can run the following command before rebooting machine and checking the BIOS option:
# esxcfg-info|grep "HV Support"
The output will determine if the machine is 64-bits compatible:
0 - VT/AMD-V indicates that support is not available for this hardware.
1 - VT/AMD-V indicates that VT or AMD-V might be available but it is not supported for this hardware.
2 - VT/AMD-V indicates that VT or AMD-V is available but is currently not enabled in the BIOS.
3 - VT/AMD-V indicates that VT or AMD-V is enabled in the BIOS and can be used.
More information here.