PC vs MAC

Okay, I am not seeking an all out slanging match between M$ lovers and MAC lovers. I want to keep it civil here. What I am seeking is a thorough comparison between the two.

Here we go …

How different are MS and MAC operating systems?

About the overall working of the MAC.
What about internet connections compared to how we use it in a PC?
What about networking? Can a MAC be networked with a PC?

What about hardware? Is a MAC upgradeable, like adding more RAM or faster CPU etc
Is it as simple as in a PC?

In a MAC O/S, what about chat programs like MSN Messenger or Yahoo messenger? Are there versions of these for a MAC?

I will no doubt add more to this as I think of other things.