Hi. I am new here as well.
I also have a Yamaha YPG-635, and I run the headphones out to a pair of M-Audio AV40's. You can get these for about $150 for the pair, and they sound GREAT! I don't know what your price range is, but I didn't want to spend more than $200 for speakers for a digital piano that already HAS speakers that I already paid for, heh.
But if this is your price range, you will be seriously challenged to find anything better, unless you buy something used. They are 20 watts apiece. Plenty of nice bass (a definite upgrade from the 635's onboard speakers) and the mids and highs are loud and crisp.) They come with all the wiring you could possibly need, even a cable to hook your mp3 player to auxilary in on the speakers so you can play along with your music. One problem I have noticed though" with headphones plugged in, the sound can still be heard faintly over the speakers. I returned them, and the replacement pair does the same thing. Oh well, I'll deal with it. Cuz it's nice to be able to play along with my mp3 player.