Rodar's right in saying you could do a whole lot better than a RealTek interface, which is pretty close to the bottom of the barrel as interfaces go.
However, you do not NEED to go out and buy a new interface. You should be able to use the RealTek without these problems.
The message you're getting may or may not be telling you the truth. Unfortunately, it's a message that can accompany many different problems that are unrelated to what formats the sound card can and cannot support.
First of all, do you have any sound out of your computer at all? Video games? Playing CDs via WMP?
If not, go to control panel and make sure the RealTek is enabled and selected as the default audio device.
If audio generally works but SONAR does not, switch to MME and see if you get audio. (It's also a good idea to verify that you're working at 44.1Khz and 24 bits; the RealTek may require a driver update to handle higher sample rates).
Let us know if that gets you going or not, and we'll offer suggestions as to what to do next.
EDIT: Oops, I re-read your post and realized your problem is with recording, not playback. Or am I reading that right?
post edited by bitflipper - July 26, 09 9:11 PM