Start a new process when memory/cpu utilisation falls

Hi all,
I am a newbie to whole shell scripting thing. I deal with computational studies and a lot of softwares which run on linux. At times, I have to schedule my jobs after some process has started. In these case, I have to use at command to schedule my job.
However, I would like to know if there is a command or script which monitors the %cpu/memeory usage of my computer and when it fall a certain value, another predefined process(job) starts running.
I have also used wait command, but that is only useful with fresh start and not when one process is already running and I want to queue another job after it.
Basicaly I want to schedule the jobs one after one.
Thanks in advance.