The Cowboys do not need the Dolphin's pick to get McFadden. Jones has already stated emphatically that he won't give up Barber, but that doesn't mean that a trade for less with say, the Raiders cannot at least be discussed. Remember who the king of the wasteland in Oakland is. Davis has made some foolish moves over the years and one more wouldn't surprise me at all.
Because of the depth of this draft, I am of the opinion that the Cowboys can better serve themselves by holding pat and using their existing picks. That blade cuts both ways though, because in a deeper draft later first round picks have more value in a potential trade scenario.
If the Cowboys passed on McFadden and he turns out to be another Peterson, there will be many lamenting what could have been.
I forgot to add that personally, I wouldn't mind seeing them get that other Smith from Florida (UCF).