my site will contain MOSTLY pics of celebrities, but I don't know whether 5GB is enough or not.
BTW, will it help more if I place just a few galleries in one page than having a whole page with lots of pics (even though I thumnail it.) Which one will take up more Data Transfer?

it all depends on how many visitors it will get and the image sizes.

add the size of the page itself and all the images on it, and multiply it by the number of visitors per month to figure out how much transfer that one page will require. if people visit an average of five pages, do that for 5 pages.

Here's (http://www.numion.com/Calculators/ServerReq.html) a good calculator on roughly how much space, etc, you might need. It's always good to buy a lil more than what you need, in case your site expands. Also when searching for a host, find one where you can upgrade hosting packages without extra cost (sometimes an email to their support can answer this).

I have another question, on that Pages per visitor and Page Size, shouldn't I do the site? Like count all the pages I have in my site and the space I have in my site?

If you think that every single person who visits your site will visit ever single page on your site then yes, if not just make a guess.