get a nice X-Fi card, crystallizer and CMSS-3D pwnzors my world, eax also is nice for games, movies and music that support it.
crystallizer enhances crappy music/sound so that if its encoded at a lower bitrate (lower quality) then it'll puff it up to a higher bitrate (simulated higher quality)
cmss-3d will simulate up to 9 speakers/sound sources in a normal 2 speaker/2 earbud system
the sound is noticeable imo