Mace
Vice Admiral
If you're gaming, most games natively have 5.1+ output modes, and all you really want is to output those 5.1 channels straight to the receiver. HDMI is easy, but S/PDIF or TOSlink require a DTS Connect/DDLive soundcard (those formats only output bitstream or 2ch audio), in which case your Xonar is the best bet because it is one of the few that support it.
Just dug through some mainboard specs(The ASUS 1366 boards(P6x series) feature a chip(ADI 2000B) that does not Feature DTSConnect or DDLive, Gigabyte GA-EX58 series feature a Realtek ALC889A Chip that does have these features(as does MSI but I have had some bad experience with their lifetime expectancy). All boards in the range feature SP/dif(TOSlink) and AC-3 out.
The money you save on a soundcard you can pump back into amongst other things a higher spec CPU. It would be a shame if you purchase an expensive High end seperate soundcard with features you'll never use.
It's just a though but something word thinking about. I used to oppose onboard sound too for a long time, but only one machine that I sometimes use for music editing features a "real" soundcard with a DSP, and that card is about 11 years old now(Terratec DMX 6fire), and still does everything it needs to do, Actually I would still be using the Maxi Studio ISIS setup I had before that, but it is incompatible with windows 2000 or XP.