I can't answer your question, but I can ask some others:

What monitoring are you doing on your servers? With your current load, what percentage of CPU and memory are you using on each server at peak times?

Most people find that Moodle is CPU bound on the webserver, providing you have taken the cost effective step of buying enough memory for your servers, so when you get near the limit, you can either upgrade your one webserver, or investigate moving moodledata to a shared file system, and buying a second webserver and a load-balancer.

But the key thing is to be measuring, so you can see which resource is going to become the bottleneck first, and how close you are.

I'm afraid I don't know anything about monitoring tools for Windows servers.