Yes, PR is logrithmic. It is very easy to achieve a PR3, reasonable to get a PR4, alot of work for a PR5, extremely tough to get a PR6 and about impossible for a PR7 (though I have seen a few).
Assume all of your links are on PR4 pages (unique domains on unique IP Blocks) with less than 50 links. According to the table at: http://www.kenkai.com/google-pagerank-table.htm
You would need:
3.5 links for a PR3
18.5 links for a PR4
101 links for a PR5
555 links for a PR6
3,055 links for a PR7
16,803 links for a PR8
While no one has cracked the actual formula for calculating PR, especially given all of the factors (links on a page, sitewide vs. home page vs. directory page with 50 or more links, reciprocal vs. one-way, etc. etc.). I’ve seen several SEO experts refer to this table as more or less accurate from their experience.