The Collaborative International Dictionary of English v.0.48:

Overvalue \O`ver*val"ue\, v. t. [imp. & p. p. Overvalued; p.
   pr. & vb. n. Overvaluing.]
   1. To value excessively; to rate at too high a price. "To
      overvalue human power." --Holyday.
      [1913 Webster]

   2. To exceed in value. [R.] --H. Brooke.
      [1913 Webster]

WordNet (r) 3.0 (2006):

overvalue
    v 1: assign too high a value to; "You are overestimating the
         value of your old car" [syn: overvalue, overestimate]
         [ant: underestimate, undervalue]

Moby Thesaurus II by Grady Ward, 1.0:

20 Moby Thesaurus words for "overvalue":
   adore, cherish, exaggerate, idealize, idolize, overappraise,
   overassess, overcalculate, overcount, overesteem, overestimate,
   overmeasure, overprize, overrate, overreact to, overreckon,
   overstate, prize, treasure, worship