I don't recommend the Asus Xonar DX for audio production. Bad ASIO and latency. If it is an internal sound card what you want, look for one of those old X-Fi Titanium (the one that came in a green-ish box). Even the guys at Image Line recommend it, as they say it has pretty good ASIO drivers, and once tweaked (in the drivers), it can deliver very good results.
That said, I would still recommend an "audio interface" instead, a dedicated external sound card with ASIO drivers especifically designed for music production. One that is excellent, and I am using right now with my FL Studio...is the Focusrite Scarlett 2i2. Excellent little USB 2.0 audio interface, flawless performance, very good audio quality, both for output and input, as it has very good Focusrite preamps...and very affordable. Much better than an audio card...it gets you to the next level, as it allows you to record if you one day choose to do so, with professional results. And that without the hassle of having to "tweak" the card to adapt it to an usable music production tool.
http://www.sweetwater.com/store/detail/Scarlett2i2Edited by Yor_ - 4/4/13 at 12:01pm