Mobo sound: how good is mine?

I have an Asus P4P800-E Deluxe motherboard with the following built in sound: Realtek ALC850 8-channel CODEC/ Audio Sensing Technology /  support S/PDIF out interface.  I'm currently outputting sound through a Logitech Z-640 5.1 speaker system but I'm thinking of an upgrade to Z-680.

So here's the question...... If I were to buy a PCI soundcard, what minimum price range (or card examples) would I be looking at to boost the sound quality of CD, MP3 or streamed music?

Thanks for any input!