Well, I can't claim any expertise on this board, but from what I can tell, it doesn't boast touch sensitivity among its limited feature set.
This looks like a pretty basic keyboard and, in general, I don't think anything from Casio would be considered "professional" (whatever that means). If your budget is really tight these inexpensive boards from Casio and Yamaha can be a place to start, but you'll probably outgrow them quickly. (As you're finding out apparently.)
How much can you spend? There are a lot of good solutions out there if you just take some time and do the research. For instance, last year, I picked up a cool little analog modeling synth from Redsound called
the Darkstar XP at Guitar Center when they were closing them out for $129.00. Read lots of reviews and keep your eyes open for close-outs. Oftentimes, last years models sell for a fraction of their original price and offer amazing usability.
The other thing to consider is what kind of sounds are you looking for? That synth I bought will be almost useless for someone trying to create orchestral music on their computer. They'd be better off with a sampler or a rompler. Conversely, if you're into dance or hip-hop you'd probably not find a sampler as useful as a beat box.
If you have a fast computer, you should look very seriously at a controller keyboard, sequencer and soft-synth solution. There are so many low-priced or free soft-synths out there you could keep yourself busy for a long time with them all. This method gives you the most bang-for-the-buck by far-especially since you already have SONAR.
Hope this helps. Let us know what you get.
Ted