You will want onboard video, else you end up putting a video card into the box and using up a slot. As far as LAN goes, if you can find an Intel board, or one with the right Broadcom NIC's, the onboard NIC may also work just fine with ESXi. Then you can combine them with stand alone NIC's to have dedicated NIC's for everything. 1 for management, 1 for VM's, 1 or 2 for ISCSI uplinks to a SAN, etc.
I'm not sure you will benefit from a dual CPU setup. Almost without question, you are going to run out of I/O bandwidth to the hard drives on a commodity motherboard or memory before you need the second CPU. Plus, if you buy 1 CPU today and 2 years later want to buy a second CPU, you may find it hard to find or expensive to purchase (since supply will be low a few years down the road).
If you can swing it, your best bet would be to put together a storage device to house your datastores. In a home environment, where fibre channel is cost prohibitive, you will be almost exclusively running ISCSI. There are some free Linux distros out there which are well known for setting this stuff up easily. FreeNAS is one of them, and OpenFiler (the one I use) is another. With this box running the datastores, it's unnecessary to really put any significant drive storage space on the ESXi box at all. By having the data residing on the SAN, you have the ability to connect multiple ESXi hosts and see the shared data.
This might be getting more involved than you want at this point. Because if you did SAN based datastores, I would recommend 2 isolated switches and 2 isolated ISCSI connections with Round Robin load balancing between them so you don't lose access to your SAN if a switch, or network card were to fail.
For whitebox setups, check out http://www.vm-help.com/esx40i/esx40_whitebox_HCL.php http://ultimatewhitebox.com/
And here is a nice build suggestion using non workstation type ports and more server based, thus increasing RAM options and such. of course, price will go up accordingly. http://www.unproductivitydefined.com...-whitebox.html