If the keywords have got high rankings in Google means that they are being searched by user in a big amount therefore the competition is high on those keywords. It is really hard to target and get traffic from those keywords cause more people like you are targeting those keywords, in that case you can target keywords those have got medium level competition.
Also just keywords are not the factors that contribute in page rank, there are many other factors like back link, content, relativity etc which Google and other search engine consider to decide about PR of your webpage.