Small Footprint PCs?

Gameface asks: "I am looking for the smallest system I can find, in quantity, for my company. We need thousands of these systems, and I'd like to ask the Slashdot community what they'd recommend. Looking for the tiniest footprint for: Case, Motherboard, CPU, RAM, HDD, Serial Port, (2) 10/100 ports. No video required, no sound, all access will be via console (serial port). No OS, just a bare piece of hardware that I can load the OS onto the HDD. I'd really like to find something with SIMMS so we can upgrade the RAM if we need to. And of course, price is very much an issue. Thanks."