Hey, David. 320 kbps is really pretty good, and unless you're really critical, you probably won't hear a difference between that and the original.
The true test on whether you should invest in a separate sound card is whether you're happy with what you have now or not. I've been more happy with my Creative soundcards than with built-in audio, but motherboards keep improving, so maybe yours is pretty good. Also, the difference in quality probably depends on whether your using the analog or digital outputs.