It will improve you sound quality (richer music) over a basic sound card. Plus an aftermarket sound card has features like dolby digital, dts, and in my case THX. They give you features your onboard sound doesn't (like optical ins/outs, upgraded EQ's and some other stuffs). The more you pay for a card, the more the features.

Basically it looks like this:
Cheap onboard sound maybe dolby digital, probibally 2 channel sound.
Upgraded sound card. Full out 5.1-7.1 sound Dolby Digital decoder, THX, DTS.

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-Justin

91 560 SEC AMG - other dogs dd
01 Honda S2000 - dogs dd
07 MB ML320 CDI - dd
16 Lexus IS250 - wifes dd

it's automatic.