From the Casio info, not sure which model the OP has but I would imagine all have USB
 
This product is not equipped with MIDI terminals. MIDI communication between the product and a computer is performed using the USB port.
 
Therefore the solo would be a good choice as it will give you Focusrite ASIO drivers and a bare bones interface for recording vocals and guitar. I say go for it.  
 
But at this point I have a small concern about the Casio as a controller. There seems to be no USB/MIDI driver for it. My Roland A 49 and Yamaha Drums will not work without installing the USB drivers. So that makes me wonder how well Windows communicates with Sonar without a driver for the keyboard.  But hopefully the interface will make a big difference in how fast VST's audio output plays. 

This audio interface will probably do what you want to do, http://www.sweetwater.com/store/detail/MTrackPlus

I'm not convinced that an interface in this price class will be a significant upgrade over your Soundblaster. But then, I haven't had to work with your current setup and I haven't used the Scarlett. All I can say is I wouldn't buy one in this range. I hate the Fast Track Pro I bought years back and would now buy an RME if I had to go external again.

Sanderxpander
buy an RME 

 THIS!!!!!

With MIDI input and VSTis, your soundblaster would probably suffice for audio playback only. Different driver modes (MME or WDM) should work but may have latency with them. Depending on whether you are doing real-time tracking or not would be the bigger issue.

bapu

Sanderxpander
buy an RME 

 THIS!!!!!

Could you please give a store (or second hand), where RME is an option for under 100$? I would like a RME at that price

The OP has already made his decision it would seem. And when it comes to under $100 there is nothing other than the Solo that I would recommend. 
Unless I'm wrong the drivers must be the same or very close to the ones used for the 2i2 and 2i4. 
From time to time I've seen a Tascam or Behringer interface for as low as $30. But they do not have ASIO drivers. The OP is looking for good drivers, not fancy in/out and he doesn't need DIN midi because his Casio has USB midi.   

Presonus, Focusrite and Tascam have units for under $100. 

I was just saying I wouldn't buy an interface of a hundred bucks as a significant upgrade over a sound blaster. I did read his post and price limit.

But I haven't worked with his set up, perhaps it will be a big change. Fingers crossed!

Wow, thanks everyone! Good discussion. Fingers crossed is right. It arrives today, but I may not get the chance to test it out until next week. I'm sure you're all waiting with bated breath.
 
Just to answer two of the points above: the Casio Privia  has a USB, and the core issue is latency. I can't get the ASIO drivers to work on the Sound Blaster, for some reason, and on WDM it's pretty much unusable. Hoping the Scarlett will be faster, and for that matter I hope it sounds as good as the Sound Blaster for playback. Having not had a new computer for a while, I was pretty stunned by the sound quality of a Sound Blaster and the cheap M Audio monitors. And anyway I really can't spend more than $100. Need to save up for Aalto. 
 
Cheers
 
Alex