Hey there. I was wondering if i should get a dac or a soundcard if I plan on getting a sennheiser hd558. My onboard sound is Realtek ALC892. Thanks for your time. http://www.amazon.com/dp/B004FEEY9A/ref=wl_it_dp_o_pC_nS_ttl?_encoding=UTF8&colid=7LZONWSXHGIP&coliid=I19ZJD4KD9KPIJ Thats the link to the headphones
You'll be fine with just onboard.
Try it and see how you like it. While it sounded 'good' to me, I wasn't happy enough with it, and ended up getting an ASUS Essence STX. Wasn't happy enough with that either, so I ended up running my Marantz 2325 to the Essence STX, and am much happier. I have very picky taste though, and have a feeling you may be VERY happy with the HD558's running on just the onboard sound. If not, you can always look to upgrading later.
Yes, you'll be fine. Don't waste any money upgrading, you'll get marginal improvements.