You can check out user reviews of the DGX-500 on Harmony Central. http://reviews.harmony-central.com/reviews/Keyboard+And+MIDI/product/Yamaha/DGX-500/10/1
If you like having lots of extra sounds, along with a decent lightly weighted action keyboard, and a pretty good piano sound, the DGX is a good buy...just make sure of it's condition (all keys work, buttons are in good shape). try to find out it's history...were there little kids banging on it, was it in a smoking environment.
I'd go with the P-85 as opposed to the Casio PX-130...I have two P-85's and they are fine reliable instruments, with a nice weighted GHS graded hammer action, very good piano sounds, layering ability (piano and strings for example), and effects (reverb)...make sure you get the optional stand.