I don't think it's a bad idea to build and tribute such monument for his wife after she died. That just shows how much love/appreciation he really have for her, plus it'll last for eternity for all future generations to see. 
I"ll have to check out, The hanging Graden.
Oh yeah I see what you mean. Their memory still lives on even after all these years and will continue to do so for as long as the Taj Mahal stands.
The Hanging Gardens is one of the original Seven Wonders of the Ancient World recorded by Greek writers. Supposedly built by King Nebuchadnezzar II of Babylon for his wife who was homesick and longed for the lush plants and tress of her native land.
However there is a controversy as to whether the gardens actually existed lol. We have found ancient tablets that recorded gardens around that place, but no one knows for sure if those tablets refer to the Hanging Gardens or some other gardens. Archaeologists also have not found any evidence to support the existence of the Hanging Gardens. They did found a foundation that could have been part of some garden, but it is too small to be the Hanging Gardens described by ancient Greek writers. Either way it's the story that matter right?