I run a Xonar Essence, which is the granddaddy of Asus' lineup. Previously, I used to think onboard was perfectly fine, but after I took the plunge on the card, I'm wondering why I didn't get it sooner.
If you have decent audio hardware, and your music collection consists of high-bitrate (320kbps) or FLAC files, then onboard will not provide the best sound. A good soundcard can really bring out more 'fullness' from the high-end, as well as controlling the lows. Details that were either muted or drowned out will be fleshed out. And people who claim on reviews that they can hear footsteps clearly in video games are not exaggerating either; gaming can benefit from a good card as it will help immerse you further.
However, my equipment takes advantage of the better quality; if your speakers or headsets aren't the best, then you will likely not find too much benefit from the dedicated card.
Still, the Asus suite which has Dolby Headphone is great for gaming and movie watching with even relatively cheap headphones. It simulates surround sound, and does a really good job of it, and provides a lot of flexibility for tweaking the 'position' of the simulated speakers, in addition to the obligatory EQ settings and other tweaks.
Driver-wise, I haven't any issues with Asus' drivers on my Win7 64-bit system. I know that Essence drivers are not terribly well supported, and have been in beta for like, forever, but the DX should be better supported given its wider use. Poking around on Google should give you an idea of what's in store for potential worries, or updates.
Looking on Newegg.com, you can snag the DX for $79.99, and that's before a $20 rebate which will make it $59.99. That's a very nice price for an excellent card.
Facebook
Twitter
Instagram