Lowest spec for todays games would be 2.0Ghz. You might be able to find some thing around that.
2.0Ghz CPU, 256Mb Ram, MX440 64Mb Video, Hard Drive, Modem, Nic, Floppy, Case, PSU, keyboard, mouse.
If i needed AU$400 badly, I might sell you the basics (Just the box) of my "SuperSonicB!tch"(see specs below). But I need that computer for leeching.