Minas Morgul wasn't the old capital of Gondor; that was Osgiliath.
When Gondor was founded and Osgiliath established, the Gondorians built two fortresses at the feet of the mountains on either side of the Vale of Anduin. In the East stood Minas Ithil, the Tower of the Moon. It later became Minas Morgul. In the West was Minas Anor, the Tower of the Sun, later called Minas Tirith, the Tower of Guard.
There is a lot of information in the appendices to Lord of the Rings, in The Tale of The Years. Alternatively, you could check out a Tolkien encyclopedia such as The Encyclopedia of Arda at
http://www.glyphweb.com/arda/
.