Basically when connecting via digital connections the soundcard will not affect the sound quality at all. The determining factor in sound quality will be the receivers internal DAC. When connecting via SPDIF the cable can be subject to some electrical interference and could cause a hum. Connecting via optical output will eliminate this problem. Both optical and SPDIF have length limitations that can affect quality. So what I am trying to say is a soundcard is pointless unless you plan on using its analog outputs.
^^ True, most soundcards are optimized for analog output, but you are still having the card do its own post-processing to the audio, so you will be getting a much better processed audio stream, even when using digital.
The D2(X) uses optical by default, not RCA coax. They come with two converters for the SPDIF In/Out ports though, so you can go either/or as needed.
The only mention I have in regards to the D2X:
1: Requires floppy power connector
2: The PCI->PCI-E bridge has some known issues with nforce chipsets [not a big issue anymore...]
I generally recommend the PCI D2 over the D2X for those two reasons, but the cards are functionally equivalent.
It will certainly help, but audio is subjective. The speaker/headphones you use also plays a part.
The biggest downside to using digital output is some other device is doing most of the audio processing. The soundcard still does its share of work [the driver layer does a LOT of post-processing on the audio stream], but analog is where soundcards really shine. For using optical output, its very hard to justify anything beyond an ASUS D1/DX [Note: only supports Dolby enocding] or the HT Omega Striker.