if you can afford it, i'd say go with the HD580s. while using a sound card is not optimal, it still will have a leg up on the HD497s...and if you ever can get yourself an amp, then they will really sing.
i've had mine for a few days now and have played them straight through the headphone jack on my ipod, via a Total Airhead (fed by the line out of the ipod), and via the headphone amp of my Creek 5350SE integrated amp.
i prefer them in the reverse order. (i plan to post a review or two in the weeks ahead after i've had more time to fully evaulate)
but that being said, i was very surprised that in spite of sounding a little less refined and controlled with the plain ipod, they still sound good...not as good as my unamped Ety 4Ps, much better than the 280s and 497s which i had the chance to hear out of my ipod. just more refined, higher resolution, or more to the point: more musical.
i just recently got some 580s for $149 from Audio Advisor and i have to say that i am quite taken with them. i had heard them many years ago. Sure, the 600s and 650s offer another step up in resolution, but for $149, i think the 580s are truly a high end bargain. they are a set of headphones you can grow into and appreciate with future options including a separate amp and high end source.
if you budget allows, pick up a pair from AA (using their $50 coupon) and you have 30 days to decide whether you agree with me or not. if you don't, they return them and only lose the retun shipping costs (AA ships anything over $49 for free)