If your motherboard has digital out, might as well use it and save your money. Your motherboard will send out the same digital signal that a sound card will. Just a bunch of 1's and 0's. If you wanted analog out, that's a different story.
If your motherboard does not have digital out or if you insist on a sound card, I would recommend the Xonar cards from ASUS. I'm a big fan of them and have been using them for a while. The best value for you would be a DG/DGX depending on what expansion slots you have available. They are the cheapest models but still better than most Onboard sound you find in motherboards. Since you will be using Digital Out, there's no advantage to getting a higher end model. It's the same digital signal.