What to look for in a MIDI keyboard?

Look at novation stuff.  The keybeds are fatar (as far as I know) and they come w/ different levels of knobs and stuff.  I don't actually use those too much, but I like the feel of my 25 Le.  And the x/y pad is great for alchemy.

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Any MIDI controller will work fine with SONAR. I would strongly urge you to try out any keyboard in person before buying, because it's a tactile device and how it feels is everything.

1. How many keys do you need? You might assume a compact 25-key controller might be best and take up the least amount of space. But as soon as you try to play a sample library that has keyswitches, you'll realize your mistake. 49 keys is the bare minimum, 72 or 88 is ideal if space is not an issue.

2. Some controllers offer user-assignable knobs and sliders. These can be very handy. You can use them to move the virtual controls on your software synthesizer. Maybe you care about this, maybe you don't. Some people consider it an essential feature. Personally, I almost never use the feature and wouldn't miss it if it was gone.

3. Some controllers have between pressure pads for percussive input. Some are tiny and can be actuated by a single finger only, others are big enough you can play with two fingers or a stick. It's a fun way to manually play drum parts. However, the sensitivity of these pads is never quite right, either too sensitive or not sensitive enough.

4. Most controllers nowadays do not have an internal power supply. I don't like wall-warts. I'd glady pay a little more for a controller that had an internal power supply. Some don't have a power supply at all and are powered off the USB bus. I don't like that at all (except for my little miniature keyboard I carry around when I travel). In the studio, I use 5-pin DIN, so I'd have to plug in a USB cable just for power. One MIDI controller that I have (Axiom 49) can be powered via USB or a wall-wart, but the wall-wart cost extra and is not included, requiring a separate purchase.

5. Speaking of 5-pin DIN...that's the old-fashioned way of hooking up MIDI devices, and most folks nowadays use a USB connection instead. I, however, am a fan of the old way of doing it. Just in case you should discover the benefits of DIN connectors, make sure the controller you choose has them. Not all do, especially at the low end of the price range.

6. If you ever end up using your controller on stage, you'll want to be able to do patch changes and/or MIDI channel changes quickly. If the controller requires you to pull up a menu in an LCD window to change the MIDI channel, skip it. Look for a dedicated switch instead. Even more important, if your controller is less than full-size make sure it has dedicated buttons for transposing! 

7. Some low-end controllers lack pitch and/or mod wheels. These are absolutely essential IMO. Do not accept a touch pad or joystick-type controller in place of wheels. Only a Moog-style pitch wheel (returns to center when you let go of it) and mod wheel will do. They should be ergonomically placed such that you can easily reach them with your left hand while playing the keyboard with your right. (One of my synths requires an awkward long reach to get to the wheels.) Wheels should not offer much resistance - on my Axiom, the pitch wheel's springs are too tense.

8. Some controllers include a basic transport control (play, stop, rewind, FF). That's a surprisingly useful feature if your MIDI controller isn't right next to your computer keyboard. 

9. Another nice feature if you use the controller on stage is backlit knobs. One of my controllers has this and I wish all of them did. (Why do so many devices have black buttons on a black background with tiny off-white labels? Engineers that have never worked from a darkened stage, that's why.)

That's all I can think of off the top o' my head.

One possible issue to look out for with the plethora of inexpensive (IE: crap) midi controller keyboards these days is velocity range....meaning - if you require a 127 velocity when you hit the keys hard, will it actually give you a 127? Some have a difficult time doing that. Not good.

It's not easy to test for that sort of thing in a store, but just Google reviews on any you might be interested in. If there's an on-going issue with velocity, (or anything else) you'll probably come across a lot of users complaining about that issue.

redbarchetta - not sure what your PC is since you don't have a signature yet. I have a laptop for my PC.
 
I love the mobility of hooking up to stuff in my studio setting if I need all the gear, and I love having the option of going elsewhere with Sonar and my laptop and being able to do most of everything not connected to any gear.
 
My special bliss in being mobile and being able to be productive in places outside of my main studio setting is having a Line 6 KB37 keyboard.
 
For a very good price point, with the KB37 I get a mobile keyboard, a fully functional audio input device (it serves really well as the external audio for Sonar), guitar and microphone inputs, and PodFarm.
 
PodFarm has turned out to actually my favorite of the guitar amp sims I have including Guitar Rig 5. The KB37 works great with Sonar, and allows me to accomplish the kinds of things you were asking about in your initial post above.
 
If I were a full-blown keyboardist like lots of the folks here, I admit this keyboard wouldn't be a concert performer.
 
But for laying down tracks, melodies, etc. it's great! It can move in any direction with octave settings, so you can also play any key range as you need.

Check out the Jordan Rudess Omnisphere demo on this page to see a secondary mini keyboard in use as a keyswitcher.

RE: Akai LPK25 ...I've got one, too. But I haven't tried using it that way. Mine stays in the computer bag for traveling. It's pretty cool, composing at 40,000 feet over the Pacific. I just wish they made a mini pitch/mod wheel controller to go with it.

bitflipper

Check out the Jordan Rudess Omnisphere demo on this page to see a secondary mini keyboard in use as a keyswitcher. 

What a great idea! I had considered a smallish live setup using a combination of two controllers, like a 61 and a 49, to handle two-handed parts. I was thinking along the lines of more flexibility and two lighter pieces of gear as opposed to one 88-key monster. I hadn't considered that the left-hand / bass controller could be used for keyswitches. That's brilliant!
 
To the OP - I have a Novation ReMote 61SL in my studio. I got it thinking that I'd use all the knobs and faders for controlling all sorts of things in SONAR and on synths and FX. Like other posters here, I ended up just using the mouse, which I then upgraded to a Kensington trackball, which is very nice and easier on my joints. (I notice in the video that Jordan Rudess uses one - so maybe I *am* trying to be like him. ) Just my experience - but your preferences may be different.
 
That said, I am thinking that the extra controls will make more sense in a live setup, where the mappings are likely to be more fixed, for example, where you may have the faders control the synth levels in a stack, and knobs controlling EQ or overdrive in an FX chain.