Low profile Audio card

I'm getting a small form factor Dell Optiplex for less than I can build system. However, it doesn't have S/PDIF, but there is a PCI slot for a sound card. I want the S/PDIF to plug into my low end Harmon Kardon 5.1 receiver. The receiver doesn't support sound over HDMI, so the new video cards that do that aren't an option.

I'm a litle confused about the cards though.

A nice card like the Xonar DX has the digital output. But does all the fancy capacitors and low SNR really help with digital output? Does that just help with converting from digital to analog, which I won't be using anyway?

If it doesn't really matter then the Xonar DG is a lot cheaper and has built in Optical SPDIF. Or the Turtle Beach Audio Advantage Micro II

Can anyone explain things for me?

Thanks for the answers. The computer doesn't have in pinouts for the sound and it's only has one analog output. The receiver does have analog inputs, but I don't think the built in sound will work for that. I primarly want it for the DTS/DD. With the DTS/DD, does quality of the chips make any difference in the quality of sound? I assume playing MP3's would work with Turtle Beach usb?

Ok, thanks for the info. I like your guide renethx. It's the best on the internet

Is there any benefit for using the analog output over the optical spdif?

Is it possible to turn up the voices on the center channel and turn down the crashes and gunfire type sounds using optical spdif or analog output?