This one’s cheap and easy, I swear:

1. A 1Ghz Dell Optiplex GX150 — the small form factor case. It’ll run you maybe $75 new, and that’s with a 40Gb hard drive and 256Mb memory;
2. A 64Mb Nvidia Geforce 440 MX AGP card, again the small form factor. If you can track one down, I’m sure it’ll cost you less than $15

Set them up with a default installation of Edgy Xubuntu, add Beryl and watch their faces turn green. And red, of course — because it is Christmas, you know. 😀