Hi Gary,
It's going to be up to your personal preference and budget ability of course.
In general, I would recommend buying the most powerful single computer you can, especially if having a very "pretty" sim is important to you. I have an older machine and I cannot turn on all the new rendering features--I would need a more powerful GPU in particular. While using multiple monitors with one computer is more restricted in terms of the view, it can also be set up a bit cheaper than multiple computers.