Hi, I only have 3 sites, 2 PR4 and one PR0. My blog has "jumped" from PR1 to PR4, but not my other site, should I get the two PR4 linking back to the PR0 in the hope to increase the ranking? Will it make any difference in the long run, (in terms of visitors and so on)? FFMG
yes it will help increase your other site's pr, but if you own both domains and their both on the same server the link will probably be devalued - but 's better than nothing.
As long as it is not overdone (exact number???), it helps get things off the ground. Getting inbound links from other sources, especially from sources of similar topic/theme, will be most beneficial. IMHO.