Noticing a difference in sound quality on my mac and my PC, I was wondering what sort of audio cards macs use. And also, in the future, with macs on intel processors, will mac users be allowed to share the same audio/video cards?
Macs don't use sound cards out-of-the-box, although 3rd party sound cards are available. SoundBlaster makes one, although their drivers haven't been updated in quite a while. I'm not sure what the exact audio processing chip is (Macs have their sound built-in to the motherboard), but it probably varies by model, since some support audio-in, some don't, and some can do optical in/out while others can't.
As for Intel-based Macs being able to use Windows-based audio/video cards, will the cards be compatible out-of-the-box? Yes, for the most part... they won't do anything at all unless Mac drivers are written for them, and that's usually up to the company that produces the card.
Macs have built in 16 bit wavetable sound built in. The sound quality of digitized sound can be improved by using a Griffin iMike. It attaches by USB. They also make a Pro version. A card is not needed for that.
It would be nice to have surround sound support, but even if you had the hardware, until it is common, the games will not make use of it.