* You are going to be using component speakers or spent a decent amount of money on replacement speakers
* You are using amplifiers of good quality (brands such as Rockford, JL Audio, Kicker, XTANT, Audison, ARC, Zapco, USAmps, Steg, Focal, Butler, TRU, Brax, etc. just to name a few reputable brands), or if your 4 channel amp costs more than $200-300, you are in the market for better sound.
* Your computer audio output goes directly into amplifiers you might be in the market.
* You desire more control over your sound that onboard cannot do (crossover, better EQ, time delay). If you want to do things with ASIO, look for a supported ASIO card and Read here:
and here: Software with good soundcard
A cheap way to do more advanced sound is an Audigy Card with the KXPROJECT drivers.
* You need more channels than 5 or 7 depending on your onboard sound.
* You have good home theater equipment i.e. you do not listen to music at home on a home theater in a box setup or boom box.