Check charitable thrift shops like Habitat for Humanity, Salvation Army, etc. They're pretty good-natured people and would likely give you a discount on an item you buy for a church on top of their already low prices. Plus that's the kind of place where you'll find a 400MHz machine being as old as it is.
I actually have a machine I could GIVE you, but I don't know what shipping would be for it. It would kind of suck for me to have to pay money to give you a free computer. See the problem??? In fact, it was the computer I was using in my own MAME machine before I started my recent upgrade. It needs a new (pci) video card, though. I pulled the old one out. It still has onboard video, but it doesn't support all the proper resolutions for the games (especially vertical games like Galaga).