Me and my wife and going to arrive in Hannover September 28th for 3 complete days. My wife is interested in seeing many Castles, I understand there are many castles close to Hannover or a quick rail ride.. Do you know if it is possible to takes tours from Hannover to visit some castles? Thank you very much.

see e.g. this thread
http://forum.virtualtourist.com/discussion-364339-1-1-Travel-0-0-Hannover-discussion.html

castles near Hannover
http://80.86.160.11/hannover/TISWeb.Dll?LieferInfoSeite(GEO||AUSFLUG?INDEX||KAT_BURG_S1)&herkunft=HANNOVER&
Most interesting is the Marienburg Castle
http://www.schloss-marienburg.com/
Actually there are not that many castles in this region. And these are rather palaces. No medieval hill-top castles. Alone for the lack of hills in the nearer surroundings of Hannover.

I reside at Hannover for two years now and usually show my visitors the Marienburg approx. 20 minutes by car from Hannover, and Bueckeburg (approx one hour by highway). But the guided Marienburg tour with one hour is rather short and in German only, and the Bueckeburg tour is very short and only shows a few nice rooms (no bed room for example). But you can take a walk in the park, and visit the burial place of former counts (pay extra). Bueckeburg has a train station, Marienburg would be better by rental car, since it is a bit remote.