There's an article on this topic at:
http://ezinearticles.com/?40GB,-60GB...ose?&id=834409
Buying a 60GB model is better than spending the extra money for an 80GB, unless you're wanting to turn your PS3 into a computer, in which you would need the 80GB. The 40GB model only has two USB ports compared to the four found on the 60GB and the 80GB models. Also, the 40GB model does not support backwards compatibility for games created for the older Playstation systems. On the other hand, the 60GB is backwards compatible with PS2 games -- I'm not sure about PSOne games -- while the 80GB has limited backwards compatibility.