Really? I didn't know that any mobo's inlcluded on board 5.1 or better audio.
Huh. Well, I always learn something new every day here.
The most common audio chip on motherboards (some Realtek thing) supports it. The slightly improved version fitted to some boards like mine has 2 extra ports over the usual 3 allowing it to work with 7.1. But anyway, it means virtually every modern PC supports 5.1 by definition, either through the mobo chip or by with a seperate card if it doesn't have onboard sound. :)
Even with improvements to onboard sound, it's still a good idea to add a dedicated soundcard to increase sound quality as well as free up your cpu from having to do the work.
My board has Realtek ALC850 7.1 onboard, but I've never used it.