I learned the hard way that even if you can install a 64-bit OS on a computer, it doesn’t not necessarily mean that you can run guests in ESXi in 64-bit.

Essentially on an Intel Platform, it has to support EMT64 and VT(and the motherboard has to support it).  For AMD, you need Rev D AMD64 processors.

With Windows Server 2008 R2 supposedly being 64-bit only, planning ahead for the future makes some older servers obsolete a lot sooner than anticipated.

Reference Here: http://kb.vmware.com/selfservice/viewContent.do?externalId=1901