Hi there
Pparks1 started me off -- for testing you can use a fairly limited White Box -- the main difficulty will be in getting the NIC to work as its quite picky. I got the INTEL 1000 PRO to work (single adapter) but depending on your needs you could use a multi lAN adapter. ESXI 4.1 or 4.1U1 (upgrade 1) are less fanatical about the hardware than previous versions.
Most onboard LANS on Home computer grade MOBOS won't work --if yours does you are licky but the Intel one is cheap and almsost every store on the planet will have them in stock.
I'd just install on any INTEL chip pased MOBO - you won't be too worried about performance at this stage so even slowish "home" computer grade SATA disks will be in order.
I'd go however for a QUAD CPU with at least 4GB of RAM
The other thing to note is that you'll need a separate Windows machine running the Vsphere Client to be able to define and install your VM's -- any old laptop will do for this purpose. I've got the VM's defined on "Data stores" on an old desktop.
The Vsphere Client is a Windows application so you will need at least initially a Windows machine to create your VM's. You CAN convert vmware workstation vm's too so if you have several you don't need to install the whole GUEST OS again.
The actual Hypervisor install iteslf is easy -- only uses about 2GB of disk space so you don't have to wipe all your Windows partitions (or Linux ones either).
I'm trying to install the Vsphere client as a Virtual machine on the server itself so I can define and access VM's directly on the server without the need for a separate computer -- but I'm still learning.
For much more professional approaches and details of peoples actual White Box hardware have a look at this site
Ultimate VMWare ESX Whitebox | Hardware: Motherboards
I'm still very new to this but so long as you can get a NIC card to work you can experiment with real cheapo solutions. Obviously in a real "Production environement" things like SCSI and RAID are important as well as OODLES of RAM and probably a lot more cores - but I'm just interested in learning at the moment.
BTW forgot to mention that ESXI 4.0 and above all need 64 bit CPU's - but I'd imagine this is fairly self evident. Enable the VT feature as well in the (REAL) BIOS if you have that option.
32 bit (as well as 64 bit) guests install OK.
Cheers
jimbo