Pretty much what Magus said...For a chart on which ones supported PS2 games (and how they supported it, via hardware or software), see the following wikipedia article and scroll down to the chart...
http://en.wikipedia.org/wiki/PlayStation_3
I have the old 60 gig with the emotion chip or whatever it's called in it, so it handles backward compatibility with hardware...They took the chip out in later models to save $$...