I use an old PCI SB Live value (4 channel) in my Epia V8000 car setup, and I reckon it sounds very good. I have an Audigy 2 ZX in my desktop and I don't think I notice a huge difference. The onboard sound on the Epia was absolute rubbish, no bass would ever come out clean, and the levels were too low. Epia M's have much better audio apparently.
I use my SB live in 4 ch mode feeding 4 channels into Kicker crossovers mostly to pull the levels up a bit before they hit the amps. Sounds great. There is background hiss when nothing is playing though.
I've tried a cheapish no-name USB sound card (admittedly with gold-plated terminals so can't be too bad) in my car and it had NO hiss at all and sounded quite good. I believe things like the Xitel pro audio link may be good for you, USB, and has RCA outputs, but USB will always have standby and CPU issues, and so I'll probably stick to my PCI.