Check out this link
http://en.wikipedia.org/wiki/Ps3 -- there's a summary of all the models.
The 80GB model might still be available in a few stores (check Amazon), but it looks like the 40GB model is the only one that is readily available.
The original 20GB and 60GB models featured hardware for playing PS2 games, while the 80GB model supported PS2 games using software emulation. The 40GB model has no support for PS2 games at all.
The one benefit to the 40GB model is that it's supposed to be manufactured with a 65nm version of the cell processor vs. the 90nm on the older models. This would mean it would draw less power and run cooler (and quieter) than the older versions.
-Rob