Take a look at the new Roland Fantom (sp?), available in several lengths (88 keys, 88 keys, etc).
It has a lot of sounds that can be expanded. I'm using a Roland KR7 (digital piano) that has a good piano sound (according to the Yamaha dealer that has just added Roland to their product line), so Roland is worth a look/listen.
I believe the Roland Fantom is also available in a rack-mount that can be driven by your present keyboard.