I am wondering about the issue of trademarking a particular name.

First of all, do I need to trademark a company that I am running right now? What would be the benefit and potential down sides of doing something like that?

Second, what about a company I plan on starting in the future? Would it be worthwhile to just go ahead and trademark that as well? This is a company/name that I expect to use throughout my life for all sorts of different purposes.

And finally, what about trademarking my own name? Has anyone done that? Costs/Benefits?

For all of these, if I successfully trademarked them, would I later be able to go to court if anyone used: company name, future company name, or actual name on a domain name or on some other sort of medium? These are names I want to protect in some fashion.

EDIT: What if someone has a domain name that would be similar to my future company name? It is not an actual company, and it has all the default information still there, looks as if it hasn't been touched. It has not been trademarked so what is stopping me from trademarking it and then stealing the domain name in court?