My computer is a stand alone 5.1 surround system.
I got an nVidia 5200 w/128 DDR ram on the video card, an Audigy 2 ZS Platinum sound card (handles up to 7.1), and Logitech Z-5300e 5.1 speakers (THX certified). I use WinDVD for my dvd player (supports surround). It sounds VERY good! I enjoy it a lot. I have a good surround system in the living room too. If I want to see a movie that my wife doesn't want to see, I go in the computer room and watch it in surround, while she uses the living room for tv.
If weight is a problem for you, this system could still be a problem. The sub is pretty heavy, by itself. But add the rest of the speakers, the computer tower, the monitor, and whatever else you have, it will be heavy.........
Good luck with whatever you can come up with!