Specs
Pentium 4 3Ghz
1Gb of RAM (it's got two empty DIMM slots)

The system has several upgrades to make it more suitable to serve as a 24/7 HDA.
Rosewill RC-222 4 SATA port card. (Paid $30)

1 SATA port card. (Paid $20)
iStorm7 hard drive bay upgraded with Vantec Stealth 80mm fan (keep your hard drives cool). Allows you to load 3x3.5" drives into the ODD bay. (Paid $34)

4 SATA Cables (Paid $32)

160Gb SATA drive (Paid $40)
The machine comes with a fresh install of Fedora 14 and Amahi 6.

Once you enter your install code, and change the passwords the machine is ready to roll.

The box has room for an additional 5 3.5" hard drives. I've included 4 SATA cables and a molex to 2xSATA power adapter as well.
I've got approximately $225 in this machine. I'd take $200 or best offer plus S&H (Manhattanites can also pick it up locally).
Save yourself the hassle of getting a box up and running, just buy mine.