I would go with none of the above options.
Auzentech is tied to PDVD9, Xonar is tied to TMT3. The ATI 5xxx cards are, as of today, not supported in either for bitstreaming. Cyberlink is working on a patch for PDVD9, but, as of yet there is no release date. Personally, as of today, I'd not buy a dedicated audio card just for bitstreaming. It's a waste. For an HTPC, if you're impatient, grab a 5850. (5870, unless you want serious gaming power, is beyond overkill for an HTPC. Most would say the 5850 is overkill, as well. I would agree) If you have patience, wait for a 57xx card. By then bitstreaming will be available in at least one player. In the meantime, if you do get a 5850, you can always use LPCM over HDMI to get 48/16 7.1 surround sound. You could do much worse IMO.