You do understand that you're setting yourself up for a world of issues by going this route, right? For one thing, it's NOT going to be cheap... You'll need a server class motherboard, dual socket being a good minimum. You'll also really want a server grade processor (such as Intel Xeon 5500/5600 or 7500/7600 series for new 5400 series are good if you can get them). You'll want the NIC to also be on the VMware HCL, not just the vm-help site. On top of that, you'll really need a RAID controller with BBWC (battery backed write cache) so that it's a true hardware RAID controller. The vast majority of onboard RAID controllers are software RAID controllers, unless they are shown on the VMware HCL as being fully supported (hardware) controllers. You're best bet is one using the LSI MegaRAID chipset (often found inside servers from HP and Dell)...
You could, also, look on either ebay or the manufacturer's outlet stores (such as the Dell outlet) and pick up a decent base system in a workstation class tower (Dell T7400/T7500 are good choices, get dual processors). You, most likely, will still need to get a hardware RAID controller (ranges from around $300 on up depending on how many drives it will support) such as the PERC 6/i...
SAS drives are a far, far, far better choice than SATA drives too. I would avoid RAID 5 unless you have plenty of drives to feed it, and can overcome the RAID penalty attached with RAID 5... You'll see better performance from a properly configured RAID 10 array compared with the same drives in a RAID 5 array (especially when talking under 6 drives). I would even go so far as to use a single drive in the host server for ESXi to reside on, and then use a SAN (not a NAS) for the VM's to reside upon. I favor iSCSI over NFS for several reasons, but test out both if you want and decide for yourself.
I have my current ESXi 4.1 host that's a Dell Precision Workstation T7400 with a PERC 6/i RAID controller, two pairs of SAS drives (146GB 15k rpm mirrored pair for ESXi, 1TB 7200rpm mirrored for the datastore), 16GB of RAM (half the slots are populated, so I can easily add another 16GB), two Intel Gb, server, NIC's (one dual port, the other quad port, not using the onboard Broadcom since it won't support jumbo frames) with a dvd-rom drive... The video card is the lowest I could get in the T7400 (PCIe card) since it doesn't need to do a hell of a lot... It is a nVidia card, which is also my preference for all systems (I actually won't purchase a system with an ATI GPU in it), followed by the Intel GPU's... This system has been running 24x7 for almost three years now... I've sucessfully gone from ESXi 3.5, through it's updates, to 4.0 u1 and u2 and now up to ESXi 4.1 without any trouble at all. My next host might boot from an USB flash drive (SanDisk 2-8GB Cruzer model) or just a single hard drive inside. That's due to the plans to have an iSCSI SAN in place before I get the second host server. Since I'll be moving all my VM's to the SAN, I won't need any real storage on the host... Plus, the additional RAID level, and multiple spindles (will be going with a chassis that supports at least six drives to start) will give better performance and redundancy on the SAN... Having a pSwitch that supports jumbo frames, flow control, and VLAN's means I'll be able to set that up right.
IF you do go with a whitebox build for the host, just be aware (and prepared for) that you could be spending a lot of time trying to resolve issues with hardware not working either at all, properly, or fully with ESX/ESXi... I had the T7400 up and running in under an hour from when I booted it from the ESXi install disc... I know people that have tried to go the whitebox route and have spent months trying to resolve issues (all traced back to hardware selections). If you're time isn't worth anything, then go ahead. Personally, I'd rather not fight a ghetto configuration (or whitebox) to get ESX/ESXi running... I have plenty of other things I'd rather spend my time doing...
VMware VCP4
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