Anything with an Intel core duo or up. G4 powerbooks are slow, always thrashing the hard drive. Also would avoid 1st gen MB pros, from early 2006, they were blazingly hot, and you'd get just a few months apple care right now. avoid Airs, i don't think they've solved core shutdown problem:
Apple doesn't sell a laptop with anything less than a Core 2 Duo at this point. Having burned through at least 4 Macbooks in the last couple years, I wouldn't buy one used or refurbed.
http://paulstamatiou.com/2008/10/26/hopping-on-the-new-macbo...
for inbox specials, refurbs, look at apple store. Also
http://www.smalldog.com/
http://www.powermax.com/