i have a website with about 200 page impressions per day with pr1. im planning to buy an domain thats is going to expire in a few hours with about 25 impressions per day and pr5 and im dmoz. what is the best solution: To buy it and redirect that new domain to my website? or to build it again, using my website rss(for news)? i know that the new website is going to loose all the rankings but it has lots of backlinks ( including dmoz) what should i do?

You won't find a whole lot of value from that new domain. The PR will disappear once the domain expires. Google became a registrar so they can catch tricks like that.

The redirect will send the 25 impressions per day to your site but if the traffic isn't targeted I wouldn't expect much to come of it expect the slight possibility of more ads being clicked. If you redirect you might be able to get the editor of Dmoz to change the url to your old site which would be cool but I wouldn't hold my breath.

i would build up the new site and make the site better

DMOZ is experiencing major issues right now and may even be done for good. Add that to the fact that it is run poorly to begin with and I would say it is safe to assume that getting that link updated is a long shot.

As for the 301 redirect and PR, well, you won't get the PR. Google removes the PR for expired domains. They do that to prevent people from hording expired domains for the sake of increases their PR. The 301 redirect will allow you to keep the traffic but that's about it. (This s one of the main reasons they became a registrar).

I say figure out which domain is better for SEO and your users (hopefully that is one in the same) and use that as your main domain.