Pretty sure the sound card will let you hear things a certain way, and more importantly, when recording into them, their quality will certainly reflect the audio quality of what you record.
As far as exporting audio, I am 99% sure it doesn't matter what card you have. The file will get processed by sonar, and how it sounds depends on what you play it on whether it is that sound card you use with your computer or the radio in your car.
With a sound interface, you want:
1. good cnverters and decent preamps if they have them
2. good low latency performance (this comes down to drivers pretty much
3. you want the interface to have what you need - i.e. enough inputs, outputs, midi capability, whatever you need
Again, the sound interface/card will not effect an export if I am correct. It will effect what you hear when you play music through it and you judgments according to the quality of that sound when mixing, and it will effect the quality of what is recording into sonar or any program via going "through" that interface/sound card.
Lance