For my money, this is a year to trade up. With T. Smith at the tail end of his career, Z. Martin well on his way to that point, Lamb on his rookie contract for only 2 more years (and if he really explodes this year you would want to sign him to a new contract next year before he has another monster year and his price zooms way way up, e.g.) and so forth, I don't see the Cowboys have the luxury of waiting for a bunch of low draft picks to develop, which in itself isn't guaranteed.
Get some quality talent on the lines NOW, for one. Strike while the iron is hot, so to speak.
I'm not enough of a draft guru to say what picks to trade,how far up to go, etc., I'll just say trade up is the best course of action this year...