I have an Athlon 3000+ for sale in the B/S/T forum for 50 bucks, if you're interested. It can be done. I did it for $300 last summer, and it did very well.
Get a cheap hard drive off of someone here at TF, and get a low-end motherboard. You can get a cheap video card, 512mb - 1gb of ram. You should be good to go. Do you have cases lying around anywhere you can get for free?
I'd recommend looking at a basic X700 from Sapphire or someone for the video card. You'll get 8 pipes, for very cheap.
Ryan