As per the title, I want to improve the sound above whats provided on my mobo. I have a Logitech Z-5400 5.1 kit (basically the Z-5500 with less wattage). I use the system mainly for games (Battlefield BC2) and watching HD movies. There only seem to be two contenders in this sector, Asus and Creative. I'm also willing to buy second hand.
Have answered own Question: The answer is the Asus Xonar DX - just found this excellent gaming card roundup - good old bit tech.
Xonar DX. i have a Creative X-Fi XtremeMusic.. and I cannot recommend it. Get the Xonar... you'll have better drivers (and a higher chance that it will be Win8 compatible)