With ESXi being free, I set out on a mission to build the least expensive ESX server I possibly could. That meant reusing a lot of gear and buying as little as possible which led me to a discovery. Intel ICH7 and ICH9 southbridge motherboards seem to work very well in ESXi (SATA included). ICH8 and ICH10, not so much. With that discovery in hand, I built a P4 3ghz with an old Socket 478 chip, 4 gig of RAM and a 500 gig SATA drive smacked into an MSI 945GCM478-L board for 50 bucks total new invested. Works like a charm with the single drive being the only real bottleneck. So, nab ESXi Update2, an ICH7 or ICH9 board and make yourself more marketable!
UPDATE:
MaRiN noticed that the onboard NIC on this board doesn’t, well, function. I forgot to mention that part 
The easy route is to buy an Intel Pro 100 NIC for a few bucks from ebay or your local computer store. A brand new Intel Gig Pro NIC from newegg will run you about 25 bucks, and then you’ll be good to go!