My blog is about Internet marketing .if i would comment on an PR7 dofollow whose niche is totally diff to my niche will i be profited in term of PR ?
Yes PR transfers from one page to another regardless of relevancy. A less relevant link may not help your search engine rankings as much though.
yes. It will help. Especially if you make your anchor relevant to your niche. For example, if you are linking back to a credit card site, call your self 'Credit Card Expert' or something like that.
But what if my site is about " Football " and i go to a PR7 Credit card site and make a comment overthere .
The PR will increase that is if you do get traffic from the comment, and from there you should get the PR-Juice
here is my question again . Please clear my doubt What if my site is a parked domain . it doesnt have any content ,will it still gain the PR ? .
If t's PR7 it probably gets some traffic aswell, that'll come streaming to your site. if you then have good contetn, you'll get noticed by other bloggers and more links will hopefully follow. Always do something like that!