I've been on the hunt for an amazing digital piano sound for nearly a decade. I still have not found anything that completely blows me away, but I've found out what doesn't work. I will share.
The most valuable thing I can tell you, is to stay away from expensive keyboards from Guitar Center- unless you are talking about playing live. That's a different story. I know your bro's at Guitar Center are going to tell you that "this" keyboard uses "samples from a 30 foot grand piano in a 3,000 year old cathedral on the top of a mountain", but I wouldn't touch it. For recording, I would strongly recommend getting a MIDI controller, and a program to create the sound. For example, I have this MIDI controller:
I think you can find them for around $300 used. Now, this keyboard doesn't make any sounds, it simply provides keys to play on in connection to the software on your computer. There are some that are much cheaper, and would do fine.
And then you will need a program to connect with the keyboard. The absolute closest thing I've found to a real piano sound, is the Abbey Road Keyboard Library:
You will need Reason to run it, which makes it more costly I suppose, but well worth it.
So, let's say $200 for a decent MIDI controller, $125 for the Abbey Road Keyboard library, and $300 for Reason. I haven't looked in a while, but the chances are, I'd pick this combo over pretty much everything sitting in the keyboard showroom at any music store. Hope that was helpful to some of you. Thanks for reading.