A sound card's a DAC (or has a DAC in addition to a few other components), external USB DAC are just a different type of DAC... The easiest/cheapest way to get virtual surround sound for PC is with a sound card, so if your old X-Fi can fit on your mobo with your SLI setup then it's worth digging out, it's probably still better than your mobo's on board audio.
External USB DAC usually don't do virtual surround (with a few exceptions like a Mixamp or a Recon3D)... If you need a new card you can grab something like a Xonar DGX or used Titanium HD for under $60 and pair it with any amp you want. Low budget USB DAC aren't really an improvement on decent sound cards, they're just more portable and/or easier to configure.
The STX is a pretty good value for a very nice sound card and a very competent built in amp, specially if you can find it closer to $160... But going with a cheaper card and an external amp might be preferable if you're thinking of getting a Mixamp later, since you can then pair it with your amp as well.
Those prices for the Q and DT 990 are pretty high btw, where are you located?
Edited by Impulse - 3/23/13 at 1:10am