Saturn Vue 2003 - Over Heated

Saturn Vue 2003 - Car over heated.  Reservoir overflowed.  Greenish muck.  Refilled with water to get it home - a few miles.  Heated up on the way.  Mechanic (whom does not work on Saturns and did not do any testing) speculated the following:

1. There was oil in the water.  
2. There may be a crack in the oil cooler.  
3. The thermostat may be the problem.
4. DEX Cool (GM brand coolant) may be the problem causing damage to gaskets.  

Some facts:
1. The vehicle was scheduled for a replacement of the intake manifold gasket a week before the overheating.  
2. No issues with overheating prior.  
3. The car did make an engine noise before it overheated but I was not driving it at the time so I cannot describe the sound.  
4. The car was only driven a few miles before the overheating took place.  
5. The car drove fine.  No issues, no abnormal noise but a slight increase in ticking, to and from the repair shops (2 miles apart) but the temp was rising over the half way point.

Do you know what the problem could be?  Any troubleshooting/testing ideas so I can make a better determination of what is going here?  

Thanks!