Many questing addons will do this for you. There are also ones that do just this (QueTip). Look through
http://www.wowinterface.com/downloads/cat18.html to see if something fits your fancy.
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"You'd be surprised how many people violate this simple principle every day of their lives and try to fit square pegs into round holes, ignoring the clear reality that Things Are As They Are." -Benjamin Hoff,
The Tao of Pooh