I did the following hardware last September or so....Atom D510 on the mobo, 2 gigs of RAM.
Supermicro 1U case, $89.99
http://www.newegg.com/Product/Product.aspx?Item=N82E16811152107
It's a cool case with the I/O ports in the front.
Supermicro Atom D510 board with dual Intel gigabit and IPMI remote module, $219.99
http://www.newegg.com/Product/Product.aspx?Item=N82E16813182238
Seagate Pipeline drive, designed for extra quiet, extra low power, extra low noise, 24x7 running in tight spaces like DVRs. It's an ideal drive for firewall appliances. $49.99
http://www.newegg.com/Product/Product.aspx?Item=N82E16822148556
Have run ESXi on it, Untangle, Astaro, PFSense, feel like doing ClearOS again soon.
How did you find ESXi on your hardware? I've tried it a couple of times.. Works great just running pfSense. I tried Untangle and pfSense, brought my d/l speed to 5-10mbit on a 50mbit pipe. Would had left ESXi on the box so i can try different FW OSs, but the built-in watchdog and ipmi sensors couldnt talk to pfSense. ipmi sensors worked in the esxi client but I couldnt get the watchdog to work.