Many presentation skills coaches are fond of quoting the "fact", attributed to Dr Albert Mehrabian, that impact of a presentation is 93% non-verbal and only 7% verbal. The non-verbal impact is composed of 55% visual impact - posture, body language, facial expression etc - and of 38% vocal impact - tonality, volume, rhythm etc.
I myself have been guilty of propagating this particular myth. Even Dr Mehrabian, who conducted the original studies, has stated that they were never meant to be applied to normal conversation - never mind public speaking.
I do, however, believe that improving posture, body language and vocal delivery can add tremendously to power of your communication and make your words, your message much more accessible to your audience. Take an all round approach - hone the verbal message and polish the non-verbal...
You can read an interesting article debunking the 55%, 38% and 7% rule at this link http://www.hodu.com/rule.shtml