I was in the same position as you a few weeks ago with headphone and sound card. After 2 weeks searching and reading reviews, i went for
HD555 and Xonar D2 and sound as really nice actually.
For gaming, sound similar to my home cinema with separate speakers as i can tell when somebody sneaking behind me

For music the same as well, clear and can hear every single instruments and voice

For film, it sounds good as well
Review:
HD 555 review
Medusa 5.1 VS HD555
Or you can go for
Creative Aurvana Live! and probably easier to find than HD 555. It's basically the same as
Denon AH-D1001 but rebranded by Creative.
Review:
Creative Aurvana Live! review
So it depends on you if you want open or close headphone. I was gonna go for Creative Aurvana Live! as cheaper than Denon itself but it's closed headphone and can get quite hot after a while and went for HD 555 instead of HD 595 as heard from users they can't really tell the difference and cheaper

and i am glad with my choise as the sound clarity from it just as good as my sound system (i'm quite picky when it comes to sound

)
I can tell that with Xonar D2, the sound just brilliant.
Hope that helps
