any one know the differences between 20,40,60 and 80 gig?

my others half brother has 1 of the first 80's and his has a flap (with various connections underneath) on the front and chrome strip where mine in a new 40 and has no flap and silver strip

whats the differences i been told i cant play ps2 or ps1 games on it either where as he can (ive not tried tho)

bloody sony why cant they just make 1 lol