What kind of switches do you have? As long as your soft phone will mark the traffic accordingly, there shouldn’t be too much of an issue. The one thing to keep in mind with soft phones is that they are dependent on the host machine being stable. If your CPU starts spiking, your soft phone might suffer performance problems.
The switch will need to trust the markings coming from the PC. This is not usually a problem on switches from the larger vendors(Cisco,HP,Juniper, Brocade), but might be an issue from your lower end vendors. The Windows 2003 servers should be able to mark CoS/DSCP values without a problem. The Cisco router will not have a problem with DSCP. Your switch should be able to map the CoS values to the appropriate DSCP values unless it is a lower end switch. The SonicWall should be able to do the same. I am going to assume that whatever model SonicWall you have, you purchased the enhanced OS as opposed to the standard.