I have similar settings to your's Ellanios, just try to find the setting that fits you the most.
And no, getting a different sound card will not increase the distance from which you are (theoretically) able to hear footsteps.
Surround sound is very helpful in CS:S, as it helps you to be able to tell where exactly a sound comes from (without seeing an enemy on the screen, I can pretty accurately move the crosshair to the enemy's head in one move thanks to it). Surround sound headsets often come with an onboard sound card, so maybe think about getting a good headset instead of a new soundcard.
As to correct Silent Sniper properly: Only the soundquality may change, so you will get more distinctive sounds and could be able to (physically) hear a coming enemy earlier.
(No warranty, someone correct me if I'm wrong.)