The Xonar D2X uses a PCIe x1 connector, finally making use of all of the unused x1 slots on the motherboards of today and yesterday. Asus said the thing delivers 118dB Sound-to-Noise-Ratio during playback and 115dB SNR during recording. The sound card comes with certified Dolby Digital, Dolby Live, DTS Connect and DTS 5.1 standards.
What makes this sound card a bit special is the presence of a secondary music processor, which alows legal "ripping" of music you've bought onto regular MP3, WMAs and so on. The trick is called Analogue Loopback Transformation, or in technical terms, the redirection of outputs from a physical output to secondary audio processor which will then record the file in the format you want.
The second sound card is also called a "Xonar", but only bears the D2 mark.