If you are concerned about ranking high in the search engines, then it isn't so important (a PR2 site can rank higher than a PR5).
If I'm looking to buy a site just because of PR, I would want at least a PR6. I think up to PR 4 is relatively easy to achieve, and PR5 can be done with more effort. Getting to six is where it starts getting more difficult (without having a larger budget).
D