I know exactly what you are talking about and its not currently possible without an onboard audio chip. There isn't a PCI Audio Card that can do real-time hardware encoding of dolby digital. The bandwidth just isn't there. Only the nforce chipsets can do it, and not even all nforce chipsets can do it. Luckily I bought the Asus A7N8X, which turns out is one of the best motherboards for this purpose. Everything I play through it comes out of the digital coax encoded into Dolby Digital. It sounds awesome. Its actually licensed by Dolby, and from what I've heard the nforce4 wasn't even able to aquire a Dolby License...
But if you want bit-perfect audio from the S/PDIF port then you can pick up a Chaintech 710 for really cheap, and I do mean cheap, I can't say the price because I know its frowned upon. Thats the card all the high end audiophiles use because it does absolutely nothing to the audio stream before it comes out the digital port. Doesn't encode to DD, but it doesn't resample the stream like Creative cards do. Any audiophile will tell you to avoid Creative at all costs.