The Collaborative International Dictionary of English v.0.48:

overemphasize \overemphasize\, over-emphasize \over-emphasize\v.
   t.
   to emphasize excessively; to place excessive emphasis on.

   Syn: overemphasize, emphasise, overstress.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

overemphasize
    v 1: place special or excessive emphasis on; "I cannot
         overemphasize the importance of this book" [syn:
         overemphasize, overemphasise, overstress]

Moby Thesaurus II by Grady Ward, 1.0:

52 Moby Thesaurus words for "overemphasize":
   accent, accentuate, belabor, carry too far, dwell on, emphasize,
   exaggerate, exhaust, give emphasis to, go overboard,
   go to extremes, go too far, harp on, highlight, italicize,
   know no bounds, magnify, maximize, overaccentuate, overact,
   overcarry, overdevelop, overdo, overdrive, overelaborate,
   overexercise, overexert, overexpend, overindulge, overlabor,
   overplay, overreact, overspend, overstrain, overstress, overstudy,
   overtask, overtax, overtrain, overuse, overwork, pass all bounds,
   place emphasis on, point up, protest too much, punctuate, rub in,
   spotlight, star, stress, underline, underscore