It's entirely possible to use the line-out on your sound card to directly feed an amplifier, but feeding a receiver would probably be better. Since you will have a subwoofer, you'll want to have some sort of bass-management which an amp alone cannot provide.

Are you thinking about surround sound or just stereo? If you true 5.1 (or greater) surround sound from your computer, you'll need to get a sound card with a digital output (which is usually Toslink optical).