If you want to game with all the effects and cool stuff, you'll probably want an x-fi card. The entire series has digital output.
Digital output is found in various connectiosn, but mostly in two:
1) RCA Digital Coaxail, this is one jack that looks identical to those found on the back of a tv for connecting a VCR or DVD player, it is generally labelled SPDIF as that is the interface (sony phillips digital interface)
2) Digital Optical, this is found in minijacks for some laptops like the new apples or portable players, but generally is found on a pc with the bigger sized jack. It usually has a little door to put the cable in, and it may be labelled TOSLINK. This is not an electrical connection like digital coaxail.