Dunamis did really well in pointing out core issues, and a technique that I highly recommend to clients. What I'll add here is that if you go the route of excerpts, and do not add unique content to those - enough that the majority of content is the unique new portion, the site will be very poorly received by visitors and search engines.
Individual articles should be at least a few hundred words of unique content. Anything less will result in those pages either being completely ignored by the search engines, or worse, flagged as either duplicate content (you don't have to have an exact copy of another page to get the dupe content flag - even if only a good portion of the page is replicated, that's enough without that extra unique content), or even possibly not even indexed at all.