Check out hydrogenaudio.org. The forum you want is here:
http://www.hydrogenaudio.org/forums/index.php?showforum=21
They specialize in computer audio. You'll find a bunch of threads on there on this topic. I bought a sound card for audio playback but didn't have a large budget, so I chose the Chaintech AV-710. It's about $30 delivered from Newegg or ZipZoomFly. The benefit of this card is that on channels 6/7 (maybe 7/8) there is a Wolfson DAC that's very good. You have to know how to set up the card so it's using this DAC, which by default feeds the rear speakers in a surround setup, for whatever reason (there are set-up instructions on Head-Fi.org's Computer-as-Source forum). But not only was I on a budget, I am feeding a 2.1 Monsoon computer speaker setup with this Chaintech card. It's much better than the integrated sound on my mobo, and as good as anything under $100. But if I were feeding my amp with my computer, I would probably go for a higher-end card.
If by any chance your audio system is not in the same room as your computer, I would seriously consider using a Squeezebox 3 instead of a sound card, just for the convenience of it. My Squeezebox outputs to a stand-alone DAC, which goes to my amp. It's a really nice way to integrate computer and audio, there's a remote control and a nice display. The quality is very good, and can be excellent with some mods.