That is an excellent soundcard, but there's an awful lot of stray RF and electrical noise inside of a PC. They're very noisy. External DACs eliminate most of that.
If you want a DAC to feed with your soundcard, there are a lot of reasonable choices out there. Before I bought mine, I seriously considered the Musical Fidelity A3.24, the Channel Islands DAC, the Bel Canto, and the Benchmark DAC1. I think all of those are excellent, and am reasonably sure I'd be happy with any of them.
I settled on the PreSonus Central Station, which I couldn't be happier with. All of these can be found between $400 and $1000, and they're worth looking into. You'll get a lot more out of your music.