Well you shouldn't need a new sound card unless you want to go that route. A USB DAC should replace the sound card and you'll be able to get all the normal sound you would from the computer via the the DAC: That means gaming, music, etc.
I would recommend
Schiit Modi (dac) - $99
Schiit Magni (amp) - $99
Mr.Speakers Mad Dog w/ Alpha pads. ($299)
(need a USB A Male to B Male cable to connect PC to the Modi: 5-10$) & (RCA interconnects. to connect Modi/Magni: 5-20$)
Cost after shipping (and tax if you're in CA) would be under $600 if you don't have to pay CA tax.
The Mad Dogs are a fantastic value for the sound you get. I used them for music and gaming, etc.
They have a fairly specific sound signature that you can read all about here.
http://www.head-fi.org/t/613576/mad-dog-by-mrspeakers-modified-fostex-t50rp-review
The newest version was just released about 2 weeks ago. Short story:
Very comfortable. Fantastic detail, response, and separation. The sound signature is flat-ish. Bass is not overbearing; Just enough and good quality. Highs are dark and roll off a little bit so if you are into a big sparkle with your cymbal crashes, you might have to get accustom to that (maybe people prefer it that way). The Mids are amazing -- Vocals/Guitar/etc are really amazing.
Work great in games. Its not an in your face slam, but a laid back detailed sound. So I find it good for hearing footsteps and not getting fatigued by over-emphasized game music/booms. If you want a movie like in your face sound when you play games, this probably won't be what you want.
Hope this helps. Good luck.