Hello. Thanks for your question. I am a professional keyboardist/pianist/producer and composer. I've played hundreds of keyboards and piano's through the years.
First thing, is your objectives. Not sure what those are. Are you merely looking for a stand alone digital piano or something more with multiple sounds, a sequencer (the ability to record songs on "tracks") and connectivity to a computer? Are you a performer, or is this just for your home?
The Casio PX-130 is on the lower end of the Privia series. I've played this keyboard several times and I am impressed with both the action of the keys and the piano sounds (that's saying something, as I was never a Casio fan). It has built in speakers, however if you want to plug it into a speaker, there isn't a 1/4 inch jack. You would have to get an adapter to plug into the 1/8 inch (the size of an Ipod headphone jack) headphone jack to go out 1/4 inch. But, that may not be a concern. If it is, you will want to upgrade to the the PX-330. The PX-130 has a few sound patches that are "Ok." Piano sounds great, electric piano's are decent, but the organs, in my opinion are sub-par. There is no sequencer on the PX-130 (well, not what you would actually call a "sequencer').
The Yamaha DGX-530 emulates many Yamaha digital piano/personal keyboards in that price range. I've always been a big fan of Yamaha digital piano's and I've played many. I've always found Yamaha digital piano patches to sound very "full." May digital piano's sound thin and just don't have the power that the Yamaha projects. The DGX-530 has a ton of sound patches to choose from. For the most part, Yamaha has some decent sounding patches.
It seems that the Yamaha comes with a stand, whereas the Privia PX-130 does not. Also, the DGX-530 can display scores (sheet music) from songs off the Internet and also display lyrics. The PX-130 will not.
Both have 88 keys. The DGX has different levels of resistance in terms of action. Also, you can adjust the timbre of Privia if you want it to sound brighter. The DGX has a 6-track sequencer, which is really nothing compared to higher-end models. If you want to get into sequencing, I wouldn't recommend this keyboard for that.
If you want more bang for your buck, you can't lose with the Yamaha. There are far more sounds and features on the Yamaha than the PX-130 and again, you just can't beat the Yamaha piano sound! If you want true piano action however, you won't get that in its entirety. Depends how picky you are regarding this. Many of the top end models emulate true piano action surprisingly well.
But...that being said. If you are into more of a professional grade digital piano, The Casio PX-330 ($650 or so) and even Kurzweil have some affordable digital pianos for under $1000.
If you have anymore questions, please don't hesitate to ask.
Edited by Hugh on 11/29/2010 at 1:11 AM EST