Seems there's no forum for this yet!?
It seems that esxi is only supported/tested on intel xeon and amd opteron cpu's...except, there seems to be quite the following for otherwise on the net:
http://www.vm-help.com//esx40i/esx40_whitebox_HCL.php
I'm afraid to test any of these things...as I feel I won't have support from VMware if the stuff hits the fan later on down the road (this is for lab and recovery work only, not a production environment).
VMware only supports the hardware listed on the official HCL at http://www.vmware.com/go/hcl
The Whitebox lists contain hints to "known as working" hardware. You will usually find help here in the user forums for issues with unsupported hardware too. (just no official VMware support)
André
Discussion moved from VI: VMware ESXi™ 3.5 to VMware ESXi™ 4
Dell and HP both have low end servers that are on the official Hardware Compatibility List and are comperable to what you might spend for a whitebox. Both HP and Dell have refurbished, open box and end of production deals. I am sure other server manufacturers have similar equipment. You can also consider off lease production quality servers which can usually be found on online auction sites or local clasified listings.