The Asus sound cards i have heard are really nice (. They even have an EMI shield to protect the sound wave from interference from Electro-Magnetic Frequencies (i.e. no hiss at high volumes through the analog jack). I personally have not heard one of these in person, but my bro has one and he likes it a lot. It does not play 5.1 except in analog.
I use an HT Omega Claro+ at this time, I am happy, but it has its quirks...
someone on the forums named REDGTIVR6 uses/used the Auzentec X-Meridian and has won a few car-audio Sound Quality competitions.
all of these are on the high-end though, there are many others that will work for much less. just research as much as you can to learn about which one suits your needs more.
If your Pioneer stereo has an Optical (S/PDIF) input, any soundcard with an optical output will work, since it is only putting out the signal in an untouched digital form (at least thats the way i understand it).
hope this little tid bit of info helps you out...and good luck with your project.