OpenMP or MPI or OpenMPI for a distributed memory cluster?

I want to parallelize a C serial code in a 100 node distributed memory cluster. The cluster consists of 25 blades with 4 cores each by infiniband. Before I just used PBS to spread several serial runs of the program between the different nodes. Now I wonder:

1. What is the best alternative in this case OpenMP or MPI or OpenMPI (at the moment I do not wish to try a mixed approach as I am starting to learn)?
2. Where can I find examples/tutorials?
3. For a simple serial code with a main for loop, can always a OpenMP/MPI/OpemMPI perform better than a queueing approach like PBS?