@Archean
The two main ones are two HP laptops, which both ironically shipped with Vista home basic, but barely run with it! They're only 1.8-2.0ghz Celeron's, with 1GB of RAM, and onboard intel graphics. They run nicely on XP Home. I've moved one over to Ubuntu already (our 7yr old uses it) and I have to say the performance is much more improved.
Its far more responsive than say our HP 6735S laptop, with a AMD dual-core RM70 CPU, 4GB RAM, (though I can't remember the graphics) which runs Windows 7 Ultimate.
But its suitability depends on what its used for. My partner can't stand Linux (she struggles with Windows), though I can never seem to be able to get her off my iMac. She absolutely loves using that. Go figure!
Ultimately, if its just web browsing, checking your emails, writing the occasional letter or spreadsheet and its used as a tool to find your local builder in the yellow pages (for example), then Linux will work for EVERYONE.