Depends. There are a lot of variables.
A high PR9 and a low PR10 are about the same thing. Also, some techs say the Google toolbar is just a rough guess of PR. The amount of links out on the page also makes a huge difference. A PR9 with 10 links out may be better than a PR10 with 20 links out.
Ideally, if a page is passing good PR, it should pass you one less than its own rank. (IE: A good PR8 would give you a PR7) Some think you need more than 1 PR8 to get a PR7, but I think thats only because most PR pages being sold arent really all that good.
One way to check what rank an index page passes is to look at its inside pages. If a PR8 index page passes PR7 to its own inside pages, chances are it will pass PR7 to you. Best way to check... Look at links out you know have been there for two months, and see what their rank is.
I dont know any 100% sure way to pick good links. My strategy has been to buy links that look good to me, and only buy them by the month. I renew the good ones, and dump the bad ones.