I use VGA for one of my monitors, and it is fine with Sonar.
But also, for about $5, you can buy a DVI/HDMI adapter, and you are in business. Just make sure you get it going the correct way, either DVI TO HDMI, or HDMI TO DVI, whichever you need.
If you have trouble finding one, just post back, and I will find you a couple links for ordering one.
Bob Bone