One more question: Google updates PR periodically, but for internal reasons it does it permanently. When I add a link, Google counts the actual link or the displayed one?
As mentioned any PR 1 onwards till PR10 will add value so no harm Google updates internally it will count but it will show externally only once for every cycle of 3 months
Any website will add value if it is relevant and does not spam outbound links. Stop talking about PR all the time.
If you are getting a backlink from a PR2 page with not too many outbound links, you should take it. You can get a PR1 due to this link easily.
Not only will a PR 2 link possible help, but it also makes your linking much more natural. I suspect that Google would get a little suspicious if you only had links from PR3 and up sites. A few PR2, PR1, and some PR0 sites never hurt in that respect.
Do you want high PR on your pages or do you want your pages to rank high in the SERPs. If you're interested in getting traffic to your site then I'd stop worrying about PR and instead try to get good related links to your site regardless of their PR. That's what will boost you in the SERPs and bring traffic to your site.
Any dofollow link is a good one and will typically help your PR. A Toolbar PR2 page can possibly pass you more PR than a Toolbar PR3 page if the PR2 page has fewer outbound links than PR3 page. And the PR2 page may already be a PR4, but Google may not have published a Toolbar PR update recently. Basing link decisions on Google's Toolbar PR is a very bad strategy. As freelistfool said, just concentrate on getting links from relevant URLs with relevant link text even if the pages have a PR0 or no PR. PR is really overrated and has VERY little (almost nothing) to do with how you rank for a particular keyword phrase. Having a PR8 page that doesn't rank for any of the keywords that people use to look for sites like yours is worthless. I would much rather have a PR0 site that ranks in the top 10 for my main keywords any day. I'll get more traffic and make more money than a PR8 site that doesn't rank well.