As per my experience, you can easily get PR5 or PR6 from strong PR7 submissions.. ( But I dont think there are 5 PR7 directories currently )
If you were able to get your site on 5 pr 7 homepages, and those pages only had about 10 outbound links on each one, then you would get pr 6, pr 5 at least. But to be honest, getting pr 5 is really not that hard to do, it only takes about 2 pr 6's and a few 4's and you will get a 5 easily. Of course timing is everything, if those links are not picked up in time then you will find yourself waiting for the next update.
Just make sure if you get in and pay for a PR7 directory that the page your link is featured on is also PR7. I have seen too many directories with a high ranking index page, but inner pages of PR4 or PR5. This doesn't make the directories bad, but you could be paying a higher value for your link than what it is worth.