Niel: I'm interested in this thread, please do post what you end up with. I'm heading towards a DVR myself, but could also pick up a low cost ex-least PC and de-rating the processor so you do not need a fan and fit a larger heat sink might also work well.
Whatever you use make sure the GPU is capable of H.264 hardware acceleration if you plan on using the PC doing the recording for playback. An old PC without this will be fine for recording, but will choke on playback.