Looking to get a Keyboard for recording Midi

Do you want the keyboard to record MIDI itself, or do you want the keyboard to send MIDI to SONAR so SONAR can record the MIDI?  Or both?

As a reminder, midi is just a set of instructions. Midi instruments typically don't 'record' in the analog sense of the word.
 
If a keyboard has a midi out port or midi over USB then it will send the instructions out to be captured (recorded) by Sonar.  The only keyboards that I'm aware of that capture (record) midi are the keyboard arranger/workstations. But then, I'm not sure so much as they record midi in or just allow you to save what you play on the keyboard to various internal midi channels for layering and arranging in the instrument.
 
Midi keyboards as opposed to controllers, typically have a midi in port to allow other gear to control their instrument library but don't typically record. Yes, they are some that do but I'm not sure that you would really want one of those if you have Sonar - kinda overkill.
 
For example, the 'keyboard' I have is actually just a controller. It makes as much sound as the keyboard I'm typing on. It sends midi instruction out to other gear so I can control the synth module or send it to a lighting console. While these pieces receive midi they don't 'record' midi. Sonar is the only thing that captures and saves the midi instructions.

As a general rule, if your keyboard specs include a MIDI out port, the keyboard will send MIDI to another location. Sonar should be able to receive the MIDI out from the keyboard and record it. As Dr Lumen notes it  is not standard for electric pianos (which frequently do have MIDI out) to be able to record MIDI (i. e. the key presses while playing) to the piano itself, but almost invariably the freestanding "workstations" do that. In any event, if the MIDI out from your keyboard is a 5 pin DIN (old style true MIDI) connector, you will need a similar connector on your MIDI interface. It is rare to have that kind of connector on a new computer, although it was common on old add in audio cards with MIDI. A simple single instrument MIDI to USB interpreter/connector can be had for a couple of bucks that will allow you to connect a DIN out to your computers USB ports. If the MIDI output from the keyboard is via a USB connection, then no physical adapter will be needed. In either case you will need to have drivers for the keyboard or interface that work with your operating system version. In many cases the drivers are "class compliant" and are already included in Windows or will load from the device firmware when you plug the device into your computer. 

am looking at these two keyboards
Casio WK-7600 and the Yamaha PSR-EW400

I have several East West Products that I will use for sounds in sonar.  If the keyboard captures the midi instructions I don't care what the keyboard sounds like thru the keyboard speakers for recording.  As long as they sound pretty good while am just playing the keyboard is fine with me.

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

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.
 

 
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.
 
 

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.

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.

ricoskyl
This is all good advice.  However, I haven't heard anyone mention that sending MIDI doesn't guarantee that the sounds you hear from the keyboard speakers can or will be triggered reliably from the SONAR recordings.  It makes sense that they would, but I wouldn't take it for granted that all of the not all of the consumer-grade keyboards will allow you to 'round trip' the MIDI files you record without some more shenanigans. In other words, like WYSIWYG in the print world, What You Play Might not be What You Get from your keyboard speakers.

The SONAR recordings will presumably be MIDI files as in your original question. If you want to actually record the sounds made by the keyboard internal synth, then you will need to take an audio out from the keyboard and record the audio into an audio track in SONAR. That latter will require an audio interface. somewhere between the keyboard and SONAR. That is a failsafe no brainer way to achieve "WYSIWYG." The downside is that it is very difficult to correct mistakes or repair performances recorded as audio compared to MIDI.
 
If the keyboard is capable of playing under MIDI control, (clue it has a MIDI in as well as out) then unless it is really odd, the response to receiving MIDI data at its MIDI in should be capable of being identical to receiving the keypress that generates the recorded MIDI data. There are a host of features (keyboard splits, octave adjustments, voice layering, harmonizers, auto tempos etc.) that can mess that up, but it should be possible in most cases to set up the keyboard response to MIDI data in to get it to play what you hear when recording. You are going to have to at least read and understand the manual for your keyboard to figure out those settings/features. 
 
Incidentally, if your wife plays piano, you might find she will be happier with a dedicated stage piano with MIDI capability. MIDI controllers, and even all in one DAW workstations generally have keys that respond more like an accordian than a piano, albeit with somewhat more flexible control of dynamics/velocity. A good keyboard (the mechanical/transducer part) is expensive to manufacture compared to the cheap electronics that actually make the sounds.  

Some examples at Guitar Center ...
 
Yamaha digital pianos, velocity sensitive, with speakers and USB.
http://www.guitarcenter.com/Yamaha/Digital-Pianos.gc#pageName=subcategory-page&N=39923+18141+1075+14640+14741&Nao=0&recsPerPage=20&v=g&postalCode=29710&radius=100&profileCountryCode=US&profileCurrencyCode=USD
 
Yamaha portable keyboards, velocity sensitive, with speakers and USB:
http://www.guitarcenter.com/Yamaha/Portable-Arranger-Keyboards.gc#pageName=category-page&N=18192+18141+1850+14640+14741&Nao=0&recsPerPage=20&v=g&postalCode=29710&radius=100&profileCountryCode=US&profileCurrencyCode=USD