You can find out how much CPU the various unix IDs under your account are using by setting up CPU resource reporting. The following wiki page tells you how to set it up:
The raw logs will give you an idea of the peak memory utilization by process. Unfortunately, I don't think it's easy to get a feel for which processes were running at exactly the same time. See:
I don't know of any way to look at total memory requirements other than by logging in when the site is most heavily loaded and watching it using the top command.
Remember that if you are killed for resource utilization you're supposed using at least 10% of at least one of your server's resources. Support may be able to give you a direct answer for how much resource you were using and thus how big of a PS you'd need.
Remember that an alternative course of action is optimizing your site to use less resource. It's often the nice-to-have addon modules that kill performance so maybe you can just get rid of them and keep your site free?
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