A3D - Aureal has quickly made a name for themselves with this quite powerful piece of technology. The earlier versions, A3D1.x, only supported four speakers and 3D sound. With the introduction of A3D2.0, Aureal takes the actual geometry of the simulated room and makes an audio version of the geometry and figures its 3D sound data from that. This gives the developer the opportunity to give their audio a high degree of realism. In the next release, A3D 3.0, there are new features that will work on existing hardware and will allow the developer to exaggerate the effects of A3D to suite their games and applications.
Sensaura (Sensaura Interactive Positioning) - Sensaura has made a name for itself with the introduction of the MX400 and the Guillemot Fortissimo. With their EnvironmentalFX (EAX compatible), MacroFX, ZoomFX, and MulitDrive effects, Sensaura has gone past the usual 3D sound algorithm. Focusing on their Digital Ear technology, Sensaura really gives the listener a great sound experience. There is support for headphones, two speakers and four speakers.
Qsound - Qsound has quite a legacy in the music business and the home stereo arena. They quickly trampled the on-line audio craze with their co-operation with Real Networks for their plug-in for the Real Player. IQ, as they call it, can greatly enhance your listening experience. They also have their own subset of the DirectSound 3D API with their own 3D sound algorithm and EAX compatible extensions. There is support for headphones, two speakers and four speakers.
With all these choices in APIs, it may seem confusing. Don't be discouraged, most cards today support either Aureal A3D2.0 or EAX (or are compatible). Either way, you can't go too far wrong with the support and the gaming experience that you will have with either API. The next set of items that you should look for when choosing a soundcard are the Input and Output connectors (I/Os) that are on the card.
All cards today have two basic connection ports, a Line in and a Line out. In many cases they go way beyond that. Most include amplified speaker out ports, some include optical connectors for MiniDisc recorders, coaxial connectors for DAT decks and home theater connections and other I/O ports. Here is a good list of minimum necessary I/Os:
Four speaker support
Digital capability
Connectors for CD audio, video, and auxiliary.
If you make sure that your new soundcard has these features, you should be fairly well set and will also be able to upgrade to new features. Upcoming features like add-on cards for portable MP3 devices and more speaker outputs (for theater like sound) are just around the corner.