the 20 gb does NOT play ps2 games.
the 60 gb played them ALL
the 80 gb played roughly 52%
the 40 gb did not play any
those are all according to the time when your choice was between the 40 and 80.
that was last year when i bought mine. i stopped looking into it becase it no longer matters to me.
sony keeps making different models. the 80 gig now, might not be the same as the 80 last year. so if you want backwards compatibility make sure you find out wether or not the system you get is capable. because i know that right now sony sells an 80 gb or 160gb package.