Vagro,

I have the PA800 with the Yamaha PSR900 and I think the combination is a winner in every way. Sounds, Styles galore and at a very decent price.
Considering you already have the 900 I would consider swapping the PA500 for the PA800. The PA3X definitely has a number of sounds not residing in the PA800 but it won't blow your socks off imho.........
Good luck in your choice,

John