Also see: http://en.wikipedia.org/wiki/Fast_inverse_square_root

And for more analysis and improvements see those papers:

http://www.lomont.org/Math/Papers/2003/InvSqrt.pdf

http://www.daxia.com/bibis/upload/406Fast_Inverse_Square_Roo...

http://www.geometrictools.com/Documentation/FastInverseSqrt....