So both my headphones sound tons better out of my soundcard.

well first we would have to know what your soundcard is because that makes a huge difference. What generation ipod might help also

Might just be that it's providing more power to the headphones.

well to answer your question, it could be that your computer is supplying the headphones with a little more power. Remember, your computer is being plugged into an outlet vs the ipod which is running off battery. Soundwise, it's hard to believe that the realtek outperforms the ipod because in my setup, the opposite is true. But that's the answer to your question, direct source of power vs battery powered. If you're looking to balance things off with your ipod, just get a E7/E9 and attach it with your ipod. Many head-fi'ers seem to be doing that combo. For me, i connect my Cowon iAudio 9 with an arrow amp that power my sennheiser ie8s. Pretty awesome sound, and i'm happy with what i got 

I don't really like the realtek onboard sound cards I've used.  I've used two computers with it, and both of them have this awful buzzing/hissing noise in the background all the time, that alternates between channels and gets worse and sometimes all but disappears.  Kind of annoying.

realtek = yuck

Realtek onboard? WTF? There's no way it sounds better than an ipod. Realtek makes some of the worst soundcards I've ever heard and bass is NOT the strong point of them.

Not all is on the volume, generally onboard soundcards performance is poor. Why not buy a separate soundcard?