P5P800 On-Board Sound Chip Setup

Lots of dumb questions but I need a little input. The sound I get from the
P5P800 on-board sound chip is not very good. I have it connected to a
"budget" set of Altec Lansing Series100 speakers (right, left and base). I
don't know if the lousy sound is the chip, the speakers, or probably both.
I would like to connect the computer to a Stereo Receiver that is connected
to a couple of good old JBL speakers. How do I make the connection? The
receiver wants a left and a right RCA input. The MB has three plugs - light
blue (line in), lime (line out) and pink (Mic in). I want two channel
stereo sound so the 4 and 6 speaker setups don't seem to apply. Also, what
does the Sound Blaster add-on from Asus add to the system and has anybody
used it? Assuming all of the above is a waste of time what is a better
approach to my problem - like what sound card do I want to buy to allow me
to play all the MP3 files I have loaded in the Media Player 11 Library
running under WinXP?