I just looked into the manuals of the Casio WK-7600 and the Yamaha PSR-EW400 
and yes, both keyboards send MIDI over USB to a connected computer, so you can record your performance (MIDI) in SONAR.
 

Cool thanks for the info. is either model better than the other.

Sometimes, I too like to just sit down and play without having to fire up Sonar.

Syrnix
Cool thanks for the info. is either model better than the other.

Hard to say without testing them.
Best option would be to get the chance to play both keyboards in a local store.
 
But if you don't care much about the internal sounds of these keyboards (because your main goal is recording MIDI in SONAR and adding VST Instruments afterwards), the only difference (more or less) would be the keybed, the "feel" and responsiveness of the keys.
This might be really important for some but one can get very used to a keybed by time and I don't think (or won't expect) much difference between both keyboards.
 

guitar center by me doesn't carry either of those in stock but they do carry lower line versions.  i might try those and see how the keys feel.  Casio says they feel like a real piano

 
From what I see, either of them would fit your needs. I'd expect the Yamaha to have a better-sounding piano, just because it's something they do well. The Casio does have a few extra features that may or may not be important to you, such as programmable sliders and greater polyphony. Flip a coin or go with the less-expensive one.
 
 

sounds to me like your wife may just want to record performances INTO the keyboard for playback, and not necessarily into the computer.  You may want to look at more expensive keyboards that offer on-board recording, or even a workstation.  You also want to decide the type of action you want.  You may want to have a keyboard feel like a real piano (weighted) if you ever want to play a real piano and not be shocked by the feel of the real instrument.  You may be fine with a synth action, which is popular for organ instruments or synthesizer.  

I have a Yamaha PSR series keyboard.  It has a MIDI-USB driver that shows up in Sonar just like any other MIDI keyboard controller.  It even has a pitch bend control.  It was one of the cheaper models, but it does have a few good sounding samples that come from the higher end synth line.  The better units have full size keyboards that emulate the 88 key digital pianos.
 
I bought it to have a portable keyboard with it's own speakers and internal sounds.  I just turn the volume down to zero when using it as a MIDI controller in Sonar.

I think I will get the Yamaha I tried a few Yamaha and Casio keyboards not the ones am looking at but similar and I like the feel of the Yamaha better.  Am the only one who is going to do any recording and it will be in Sonar.  Wife just wants to play no recording. 

Also, check that the keyboard manufacturer has drivers for your operating system. I expect that both of these would, but if they're near the end of their sales lives, they may not.