Prevent Multiple VM's from running

I have several VM's set-up, and each one works great. However, I only have 4 gigs of RAM and running more than one at the same time kills my Mac. So much that I usually have to forcably shut the machine down to stop it.

I don't really have a reason to run multiple VM's at the same time, so its perfectly fine to open one, suspend it, and then open the next one when I need it.

The problem is that usually I start an app that launches a econd VM before I realize that one was running the background idle amd not suspended.

Is there any way I can configure the VM's so that when I try to launch them, they warn me that another VM is running and will kill the machines.

(I've tried all the options in "suspend on close of window, etc." and I find that none of them work correctly. With them set to Suspend when window closes - they stay open no matter what)