Ok, here it goes, if someone with more knowledge sees I'm wrong please feel free to point it:
1) Digital Out (Optical, Coaxial, HDMI) sends the audio in digital form, meaning your reciever's/speaker's DAC would decode it, so it's not much difference between cards using digital out.
2) Via HDMI you can send uncompressed PCM up to 7.1 (depends of the revision?) or you can bitstream (send lossy Dolby and DTS, or lossless DTS-MA or TrueHD if your reciever supports it)
3) If you are using optical or coaxial, you can send uncompressed PCM up to 2 channels, and bitstream lossy formats only (Dolby and DTS)
4) Analog output means your using soundcard's DAC, in this case the quality of the card matters
So
* The Xi-Fi HD only has analog outputs for stereo, so there is no way to have lossless 5.1 or 7.1. You can use your reciever's/speaker's "effects" to expand 2 channels uncompressed audio to the other speakers, but in my opinion is not as good as JRiver DSP options. And you won't be able to use Reclock or Video Clock without losing quality.
* I recommend you a proper reciever/amp and quality speakers. Most of recent video cards already have HDMI and supports audio, so you dont need to invest in an expensive card if you are going trough HDMI + reciever route
* If you have a limited budged the Z-906 could be a start, but as an owner of a z-5500 I dont recommend you to go this route (from reviews I have read the z-906 are a z-5500 with different pod and an extra optical input). The sub is great. but this speakers have a hardware crossover at 125hz wich you can't change or remove and the satellites are weak on mid frequencys. Is this bad? well for most users here yes, sounds boomy and loud, but definetly are enough and more for most people, the only way to be sure is to go to the store and try them yourself before buy.