ubuntu is a nice os, simple, useful, got all your agents need installed by default. Softphone i prefer xlite, who also got support for linux. Maby not the most stable versions of xlite i tryed, but better than most of the others. I have only tested this components, maby someone ells use the for professional. All our agents uses windows and xlite/eyebeam, and it works very nice.
We mostly use headset, connected to computers sonedcard, with softphone, works pretty good. Soundquality is okey, and you can trust it almost as much as you trust microsoft