What inputs do your amp/receiver/speakers have?
If you're using a receiver with HDMI, you don't typically need a sound card and you can just output digital audio using your video card.
If you're using speakers or a receiver with analogue 5.1 inputs, you can just use those. This is rare on receivers, though some of the older higher end models had it (e.g Cambridge Azur 540r) but quite common on those PC speaker kits.
If you're using a receiver with optical inputs, chances are you can only get stereo down the optical cable. In this case you'd need a sound card that supported Dolby Digital (or DTS) encoding.
Things get more complicated if you want EAX/ALchemy support.
No idea on Linux, not a Linux gamer.