Hi seanbaker,
It kind`a goes like this, If it has "speakers built-in" then it is almost always an "arranger KB" with the known exception of the 9000PRO. An arranger can do many things [ie: KB splits/ left & right also auto acomp. breaks and fill-in`s. A "work station" can do spilts as well but usualy does not have auto acomp. with breaks and fill-in`s unless you program them in yourself. Think of it like this, you turn on your psr500 and start playing a song, you pick a rythum or style chose a "voice" and your playing!! Now with a workstation if you wanted to do the same you would program a style and acomp. go back to the beginning of the song and play the "melody" in real time. The 9000pro allows you to do either [ie: you can add your choise of speakers (and subwoffer) and play it like you would your psr550. Or you could program your styles and accomp. , and then go back and play the melody in real-time. The 9000pro is unique where it allows three spilts so you could program your styles and accopm. then play in real-time 3 different instruments. If you follow this link it will take you to the 9000Pro web-page at "Yamaha" : http://www.yamaha.com/cgi-win/webcgi.exe/DsplyModel/?gPPK000059000+PRO
I hope this helps going from a psr500 to a 9000pro might be a "steep learning curve" but I`m sure that with your psr500 experiance it won`t be to "steep"
Best of luck, and keep posting we`re here to help (someday I `ll be asking for your help ) Keep in touch and good luck
jedi