@Kental2: I think that a really good project manager doesn't really need to have an understanding of how long it will take to write feature 'x'. It's the developer's job to know how long it will take, and it's the business analyst's job to properly document specifications and requirements so that the developers can properly estimate.

Where I work, we're about half way through a multi-year implementation project. The project manager started out with no history in IT. However, he had a record of the projects he's managed for some Fortune 500 companies and the military. Watching him work, I realized that he didn't really need a solid understanding of the nuts and bolts. He abstracted the high level components and worked at that level to make sure that the developers, DBAs, business analysts, customers, and vendor all kept moving along.