I don't have any site info for you, but some key things to decide on would be:
If you are a decent piano player do you want realistic keyboard action (weighted keys)? Do you want sounds - or are you planning on triggering plug ins? If you want sounds - what musical genre? Do you need hiphop/dance & trance sounds or do you want accurate sounding accoustic sounds (piano, organ, brass, etc)
If the keyboard will also be your main sound source which you plan to sequence via Sonar - then you want the most polyphony and multi timbral you can get (ie: several sounds playing at once).
Do you want to tweak sounds to create your own vision or will you be satisfied useing the preset sounds.
At a minimum you should get 61 keys (if you arn't really a keyboard player and you plan to sequence one part at a time you don't really need 76 keys and you certainly don't need 88).
I hope that helps for a start.