I'm a firm beleiver in measurement-based audio testing, and I can confirm that depending on your computer environment and speaker setup an internal card (even one like the ST) can be a hit or miss affair.
I happen to own an Asus ST and H6, and I can tell you that the card is not particularly well-isolated from electrical noise inside PCs, such that when I started using it to drive power amps, I got really dramatic (and measurable) static out of my speakers when nothing was playing. As in crunchy variable-pitch whining loudly audible from six feet away (most noticeable during quiet parts of music or when nothing was playing, but sometimes even audible over the music). It would change volume and pitch based on hard drive and cpu activity. Changing which molex power connector the card was connected to inside the PC changed the volume of the noise, but even with the "best" molex it was still quite loud.
To be fair to the card, the Asus was hooked directly to the inputs of a power amp and the speaker it was connected to was extremely sensitive (113 dB/1W), but if it had been performing anywhere near its rated SNR (124dB) there would have been no audible noise whatsoever. I know that it was not an issue with the card itself, as in another computer system that I use for testing, hooked up to the same speakers, the exact same Asus card had a much quieter, but still audible, noisefloor. Hooked up to much less sensitive speakers in a more hospitable computer, the ST is actually quite nice and I still use it.
Which means the card is just not very well isolated from electrical noise. Depending on your computer or your planned uses, it may not be an issue for you, but there's no way to know until you plug it in. After trying multiple internal cards with the first "noisey" computer (without success), I tried several external DACs and all of them (even the quite cheap Fiio E7) outperformed the Asus in terms of noisefloor, which should not have been the case based on the rated specs.
@OP There was a recent thread that might be of interest to you, where this question was discussed (to include the Asus ST):
http://yabb.JRiver.com/interact/index.php?topic=90593.msg622329#msg622329For a high quality 7.1-capable external DAC, I can personally recommend the Steinberg UR824; others on the forum have been quite happy with the Focusrite 18i20.