it really depends on what the site is made of... is it 1 page or 1000 pages, whats on the pages, text, images, flash, etc.
on
GetTipsy.com there is over 800 pages of content and the site (while still barely 2 months old) is doing around 3000 uniques/month - and has yet to go through 1gig of bandwidth/month while other sites i run like
PimpMyCase.com does around 2500 uniques/month and goes thru 3gigs bw/month because there are more images on it. the forum i run,
iPhotoForum.com which sees between 12-14,000 uniques a month is currently pushing 13 gigs of bw/month because of the sheer number of page views (close to 500k/month) - and again thats mostly text...
so it really depends
i wouldnt say go jump to a dedicated right off the bat... any good web hosting company should be able to offer you shared hosting to start and then upgrade on an as needed basis. i host all my sites on
towardex.com which offers everything from budget hosting to co-lo and dedicated servers