What's your budget?
I mixed up my office listening playback a long time ago and ended up running PC > M-Audio Audiophile 2496 digital out > Mytek Stereo96DAC > AKG K-501 headphones for quite a while. I think the Mytek sounds great. And I really like the K-501s. But I eventually grew weary of wearing headphones all the time, even though the circumaural K-501s (the cushion surrounds the ear, doesn't rest on top of it) are mighty comfy. I picked up a pair of monitors a while back (thanks, Heath!) and now run PC > M-Audio Audiophile 2496 digital out > Mytek Stereo96DAC > Alesis M1 MK2 Active Monitors. I'm glad I did - while I really like headphone listening, I prefer monitors.
FWIW, I also have a pair of Sony MDR-7506. And while I like the Sony's, IMO the K-501s blow 'em away in both sound and comfort. Not surprisingly, since the AKGs are open and the Sony's closed. Also, while the Sony's are also considered circumaural, I'd classify them as a circumaural / supra-aural hybrid, i.e. they rest both around and on my ears: the diameter of the oval pads are too small to encompass my ears without parts of my ear touching the pads; and the pads aren't very deep, so my ear also hits the soft fabric on the inside of the cup. (And no, I don't have particularly large ears.)
Oh! As for the soundcard - I'd go with whatever is least expensive and accomplishes your goals. An external DAC makes a big difference, I think, so if you go that route - anything with a digital output will do the trick.