My personal emergency PC is a Sony Vaio C1 Picturebook. The VFK model with bluetooth. 650Mhz, 128mb of RAM (upgradable to 192mb) and a standard, easily replaceable 2.5" ide hard disk.
Not the speediest machine in the world but I gave it a bit of a kick by installing Xubuntu. The only downside I'd say is the size of the screen. At 1024x480 I sometimes have trouble fitting entire dialog boxes onto the sceen. However I think there's a better version... Possibly the C1M, that had an 800Mhz processor, 256Mb of RAM and a 1280x800 resolution screen.
The size of a hardback book, I'd heartily recommend it.