generally you have two good ways of connecting everything up.
buying a receiver and outputting via dvi/hdmi from your video card to the receiver which then processes audio and has video passthrough to the tv.
buying an amp and connecting the pc to it via splitting the signal off a hdmi/dvi cable, via spdif on your mobo or sound card or via composite/component cables using the propper plug adapter.
what you need to figure out is what output you will be using and if you need the system to be able to support additional inputs besides the pc.