It is kind of hard to provide specific recommendations without knowing specifics about your environment. Resource Pools are one of those things where there really aren't any "right" answers. But, I can provide a couple of general "tips":
If you have a critical VM that needs "x" amount of memory, then of course you would provide this VM a memory reservation of "x" Mb of RAM. Then, this VM would always be guaranteed "x" Mb of PHYSICAL memory. Keep in mind that if you provide a VM with a memory reservation, then this this physical memory is NOT available to other virtual machines, even if the VM with the memory reservation is powered-off.
Another thing to keep in mind is that even if a VM (or its Resource Pool) is configured with an Expandable reservation, set to "Unlimted", the VM can never have more memory than the amount with which it is configured. This is not the case with CPU - if a VM's CPU is set to "Unlimited", then as long as there is no contention for CPU resources the VM will have "access to" all of the CPU cycles of the physical CPU on which its vCPU is running.
For more information, I would look at the Resource Management Guide:
http://www.vmware.com/pdf/vi3_esx_resource_mgmt.pdf
###############
If you found this information useful, please consider awarding points for "Correct" or "Helpful". Thanks!!!
###############
Under no circumstances are you to award me any points. Thanks!!!