I'm happy for you that you have found a DAW that allows you to work with an onboard soundcard.  My advice would be to stick with it.  If you move to Sonar, Cubase or any of the others, they're all going to need an external audio interface.  

I have no idea how you're able to get the performance you're getting at present, maybe it's because you're not recording?  When you use the soft synths, are you playing a midi keyboard in or are you entering onto some kind of piano roll?  I ask because with the former, I would expect you to notice latency issues.  Maybe using ASIO4ALL as recommended by Cheez would allow you to not use an external audio interface if you're only using a piano roll to enter your notes and not recording audio - I don't know.  I was in a similar position to you but I didn't risk it myself.  I don't record audio, I do use a midi keyboard but I wouldn't dream of not using an external audio interface. 
 
What has prompted you to move to Sonar?  I'm not familiar with Mixcraft 6 Pro Studio but if it's giving you what you need, why move?  

The price of moving to Sonar or one of the other big players is not only the software itself but the audio interface that they all need.  That's my understanding anyway.

All the best ...

Edited:
 
why am I wasting my time here-bye

cheez


WaveRT is not bad. Compared to the older drivers like MME/WDM, it's certainly much better in terms of latency. I've started from the era of using MME for DAW. I've used all the drivers. And ASIO still beats WaveRT. There's no competition. That's not to say in any way that WaveRT is not good. In my humble experience, it's just that ASIO is better. That's all. 

no point to say that.. the OP obviously thinks his realtek is used in pro studios all over the globe ... or maybe they upgraded to SB live ;-) 

and wrongly assumes cubase never gets updates.. funny what are the update patches sitting on my hard drive for ? I mean, what do companies like cakewalk , steinberg, roland , yamaha ,rme and manye more know about pro audio 


O/S makers don't exactly think of music makers first.. and that hasn't changed over the years...

although apple does to a point,  only due to logic tie in