External DACs will 90% of the time (avoid the realy cheap ones) produce better sound than internal cards due to them using digital signal from the PC to the DAC itself, asuming you run through like USB. This means that the interferance from internal components are eliminated (Sort of but here it gets realy complicated.). The DAC it self can create interferance since it converts digital to analogue (Hence the name DAC, Digital to Analogue Converter). The soundblaster X DACs are realy good, i have used the Soundblaster X G6 for years and being one of those who can hear high frequenzy noises and background hizzes easyer than many others, I can't say i hear much static at all.
However, if you are looking for a decent DAC, I would go more on features than brand, if you don't need most of the features that, for example the G6 uses, then there is no point buying the G6, you can go cheaper and still get a good DAC from a reputable brand. Same the other way around, you can go allot more expencive but DAC vs cost is realy diminishing returns when it comes to sound, Features is a different storry though.
Then we have the part of sound profiles and what nots but then we are starting to talk about personal preferences.
And don't get me started on the placebo stuff