A lot of people have been recommending Xonars based on their excellent performance with positional audio over headphones.
Just thought I'd mention that I'm pretty sure this isn't the case with the Xonar DS. .
Dolby technologies are not mentioned in the specs. Hence no Dolby Headphone, no Dolby Digital Live and no Dolby Virtual Speaker.
The DS should still make a good HTPC card for use with DTS receivers. It won't work with Dolby Digital receivers or give positional audio (other than plain stereo) over headphones though.