Not sure there is any acurate way of predicting PR.
But is you submit a sitemap you can look at the Page Stats and you get a little graph showing the distribution of live pagerank on your site. So this is the current PR not the toolbar PR.
No one knows for sure what the pageranks are for each category (low, medium, high) but I think the common belief is 1-4, 5-7, 8-10 respectively.
Probably going to be an update in the next4-6 weeks so then will see.
Cant say that pagerank is of any real use other than for measuring the strength of outbound links.