here is a link which will help you little what u need for L room
Surround sound card buying guide - CNET Reviews
This forum will help you understand what audio is all about.
India's Audio Video Home Theatre and Hi-Fi Discussion Forum
its as important as the video which we see it take the fun to next level .
& i leave the rest to you basically these day i am also planning to change my receiver and speaker system . i have Logitech z5500 as well has onkyo HT package.when i purchased Logitech i was also noob but after a little research i came to know what the difference in pc and AVR sound & most amazing thing is that it has a next level which u can reach by investing on some good power amp for stereo sound.