Thank you so much for your responses. I'm looking for a good digital piano primarily for my 8 y.o. little brother. It would be good to get him something he can grow into. I considered the Roland FP-7F but hesitate at the price (even though my dad is paying for most of it). Since Voxpops (or anyone else) has one, what do you think? Is it worth the extra cost?
Also, we thought about the Kurzweil Mark Pro ONEiS. That seems to be selling at a great price right now and costs about the same as the Yamaha DGX640 and less than the Korg LP350.
I would say that, if your dad is feeling generous, the FP-7F is definitely a piano that a student could progress on, and is worth the extra. I certainly would not recommend a DGX for a piano student, however young. I have not played the Kurzweil Pro ONEiS, but if it is like other Kurzweil boards, the piano-sample implementation is not as good as in Roland, Yamaha or Kawai.
These are the lower-cost digital pianos with internal speakers that can currently substitute for a real acoustic with some degree of authenticity.
Casio Privia PX-130 (or better yet, the latest: PX-135)
Casio Privia PX-330 (same piano samples as PX-130 but many more features)
Korg LP-350 (same electronics as SP-250 - not as sophisticated as Yamaha P155)
Roland FP-7F (better action)