ESX has a strict hardware compatibility list. It can be installed on a nonsupported hardware, but be prepared for not-so-smooth ride. We don't recommend using nonsupported hardware. If you need to test, you can always install VMware in VMware Workstation 6.x.

Some hardware vendors may also offer support to certain hardware by themselves, so a product may be supported by a hardware vendor to be used with VMware, but not by VMware itself.

ESX Systems Compatibility Guide (server vendors and models):
http://www.vmware.com/pdf/vi35_systems_guide.pdf

ESX IO Compatibility Guide (NICs, SCSI controllers, FC controllers):
http://www.vmware.com/pdf/vi35_io_guide.pdf

ESX Storage/SAN Compatibility Guide (disk systems):
http://www.vmware.com/pdf/vi35_san_guide.pdf