Well I just read that external links acctualy lower your PR. So I was thinking if I would use rel='nofollow' for all external links (and maybe even some internal) and keep my PR on my site. Is this allowed or would google penalize me for doing this? What about SERPs? Do external links to good and relevant sites help you?
First of all NEVER use nofollow on pages in your site. You want your PR to goto all the pages. It is beneficial and can hurt you if you don't. Secondly be careful using nofollow. Don't do it just to garnish all your PR to yourself. If the site your linking to is relevant and a great resurce give them a follow link. It will help you. You should read this article: http://www.1stsearchenginerankings.com/2006/10/10/how-pagerank-is-passed-from-one-site-to-another/ It should help you understand how it works.
I cant testify to knowing much about building PR but is it not the case if you add a nofollow to external links, the other site does not get credited? Im only saying that because I saw a few PR 6 sites selling links but having nofollow. My theory is if you are gonna link it - let google follow it
About internal linkig I was thinking about just some links like RSS link and maybe some others so I would have the best PR on main page and bad PR on other pages. About external: Are you saying that google will check the link and if it's relevant to my keywords give better SERPs? I read the article, but I doubt. And I don't want to lower my PR for nothing. And what about _blank thing? I see this forum use it. Does links in this forums give "vote" or not?
I link out to any site that is relevant––particularly authority sites––and Google has been very kind to me. The internet was built on links, and IMHO not linking to others is a very easy way to show Google that your site isn't a part of the larger community around your niche. Linking out certainly hasn't hurt me any!
External links do not leak PageRank, so don't worry about it. The only reason to use nofollow is if you don't want to give any credit to a site you don't trust. Adding target="_blank" will make no difference - this is for browsers not bots. You can get a lot of backlinks from forums. They are often not related to the content, and therefore have a low value, but they are backlinks none the less. Backlinks from forums can sometimes help get your site indexed faster. Cryo.
Acctualy it does leak your PageRank. Why? Well you give "votes" to other site and that means that you have less votes for internal links. And if your internal sites get less votes that means they are less inportant. And when this internal sites links back to homepage they give less votes because they are less important. Result: your homepage has lower PR (because it has less votes) Well here is a long article about how pagerank works: http://www.iprcom.com/papers/pagerank/. For target="_blank" i don't know where i heard it. I guess it's not true.