Is there a particular reason for "wanting" a sound card? Unless your recording something, or your hardware isn't working properly, I would rather invest in some quality speakers and receiver than buy a sound card. You won't hear a difference from the sound card if you are using $20 speakers, but you will hear the difference from a good pair of $150+ speakers.
Apart from that the D2X seems outdated and was superseeded by Xonar. The only other card that I know is the creative X-Fi which I've used personally, but ever since I switched to windows 7 when I bought an SSD I've had nothing but driver problems and ended up trashing the card and going back to onboard. And yeah I couldn't tell the difference in sound quality with $500 stereo speakers. But results may vary! Your audio chipset might not give you the fidelity you want in which case go for it