The new version of VMware ESX (vSphere 4) will run only on 64-bit CPUs hardware .
Mostly the new processors arround are either 32- or 64-bit. Today almost all enterprise servers manufactured today have 64-bit CPUs but you might want to check some of your older servers if they are 64 bits capable or not...
VSphere, however, will run on only 64-bit CPUs.
For vSphere to run, Intel CPUs needs both EM64T and also Intel VT support in the CPU and also int the BIOS of your server. EM64T is technology from Intel and VT is for Virtualization Technology.
Utility to determines if a host is VT-capable or not:
There is a utility called VMware CPU Host Info if you want to check your processor if it's 64 bits. The utility runs for exaple on a workstation, and it connect a vCenter Server and report on each host's CPU capabilities. However, this utility determines only whether a host is VT-capable or not or whether it can run 64-bit guest OSes.
Another utility for checking a CPU:
This utility is a ISO file that you can get on VMware website. You will be able to identify the CPU that supports your host and and also you will know if it supports 64-bit long mode and 64-bit guest OSes. So You burn the ISO file to a CD and boot your server on it. Get the Iso here.