Hiero, it's a good question, and one that gets a lot of debate around here. A number of folks think a WR will get chosen, although opinions vary. LB gets mentioned a lot due to the shift to the 3-4. Quarterback comes up often in the ever popular "Henson is a bust" threads. Following those come OL and safety (though not usually 1st round). I may have missed something but those seem to be the most popular.
This year there were four OL types chosen in the first round. Most 1st round picks are skill position players (and I'll include DB's in that category). Based upon that measure, it seems to me that Dallas will choose a WR or LB in the 1st round assuming we are correct in assessing the need. A TO signing could change this of course. Take my opinion for what it's worth. Other folks on this board are much better at this than I am.