Similarly to ESX 4.0, ESXi 4.0 is a 64-bit hypervisor. This means that the underlying hardware needs to support 64-bit operations in order for ESXi 4.0 to work. As you know, 64-bit instruction sets allow access to more system resources, such as memory and storage, as compared to their 32-bit counterparts.
While ESXi 4.0 does not work on older, 32-bit servers, it has a much larger hardware compatibility list than ESXi 3.5. You can look for ESXi 4.0 compatible systems here.
As of writing this blog, over 60 vendors have participated to deliver
137 servers qualified and certified for ESXi 4.0 (embedded)
663 servers qualified and certified for ESXi 4.0 (installable)
Comparatively, there are 795 servers on ESX 4.0's hardware compatibility list.
All mainstream, high-volume servers are in the ESXi 4.0 hardware compatibility list (both for embedded and installable form factors). Here is partial list of current server vendors that certified systems for ESXi 4.0:
AMAX Information Technologies
Cisco
Compusys
DELL
Fujitsu Siemens Computers
HP
IBM
Intel
Lenovo
Rackable Systems
Samsung
Sun Microsystems, Inc.
Unisys
Verari
Wipro
A final note: When installing ESXi 4.0 on a 64-bit server, please ensure that the BIOS settings allow that server to run in the 64-bit mode. Many servers are 64-bit capable but are set to operate in a 32-bit mode.
