MOSIX allows for process interuption and rescheduling on a different node without any coding changes. This allows for better load balancing of batched jobs across the cluster. MOSIX requires patching of the linux kernel to do this magic. I don't think it runs on any other flavor of UNIX. There is another software program that does a similar thing as MOSIX, but does not require any kernel changes and runs and many flavors of UNIX and Windows (partially). It is called Condor. http://www.sns.ias.edu/Main/linux/condor/
MOSIX and Condor work well it you have many independent jobs that you what to batch run on a cluster.
If you are trying to do complex computions, such a matrix inversion, finite element anlyzes, then MOSIX nor Condor is what you want. You need to compile you programs to using PVM or MPI.
PVM aka Private Virtual Machine requires you to link to libraries and run you progam under a PVH shell. MPI aka Message Passing Interface requires you to link the the MPI libraries, but you don't need to run the program under an special shell. I have been told that MPI is the more modern approach. For what that is worth.
Here are some usefull links.
Hope this helps.
I am curriuos. What are you planning to do with this cluster?