Explain what you mean by "clustering"?
I am guessing I need enough RAM to run a virtual OSX.
What virtualization software would best accommodate this?
Huh? What do you think "clustering" means?
No, it's not making a bunch of computers act as one big virtual computer. AFAIK there is no such thing.
There is no need for any virtualization software or more RAM. All you need to create a "cluster" is some quantity of computers, and some network to connect them. And some software that is able to use the distributed resources.
There are many applications that can work cooperatively with the same application (or companion "server") running on another computer. For example, many popular database servers, many video rendering applications, DNA sequencing applications, and even Xcode (which can farm-out parts of a big compile to multiple computers).
There is nothing "special" you need. Other than some "clustering aware" application.
Maybe you are confusing clustering with multi-core or multi-chip architectures, where several cores/chips in the same computer access common memory. Not the same thing.
The various "
@home" projects (and similar) are a form of clustering (aka distributed computing).
https://en.wikipedia.org/wiki/List_of_distributed_computing_projects
While some "clusters" have some specialized hardware (e.g. very fast networking, or even shared access to memory across multiple physical computers) that's not a given. And there's none of that specialized hardware that is any option for your Minis.
Here's what Wikipedia says clustering means:
https://en.wikipedia.org/wiki/Computer_cluster
It's basically all about software that knows how to use the resources of multiple computers. You could make a Beowulf Cluster. If you had some task for it it do. But it won't create a "super Mac" that makes everything on your desktop run faster.
https://en.wikipedia.org/wiki/Beowulf_cluster