I bet those XP's can out perform even your dual opteron! Go on, grab a copy of cluster knoppix and set up your own local grid
why.....mmm.. because its possible, and kernel compiling in 1minute sounds good, or actually install gentoo in less than 30mins, I wonder if UT2004 would run over a cluster?
Or actually make use of http://distcc.samba.org/
Or, perhaps a bit of both, donate a box if you think it would be more use to others, then keep one back for taking online backups of your linux and win32 box's (if you've got both). This software has a web interface and, for me, takes backups of changed files every hour from 2 windows + 1 linux box, just users home files.
Or spend £60 on a TV-Card, maybe underclock the slowest unit, lower the core voltage, get a silent cpu heatsink+fan and setup a PVR. MythTV, or if you prefer, kaffeine is excellent and now supports streaming media / broadcasting across the network. So you could get a wirless card, stick the box in the loft / anywhere out the way, and broadcast your DVD's, DIVX, OGG, DVB data to other PC's in the house.
The windows software for this card is terrible, hideous, just plain nasty, thankfully kaffeine and MythTV are far better.