Peachpurple, if you understand what the "summary" is for and why HubPages limits it to 200 characters, then you will know what to write.
When you are searching for something in Google, Google will display a list of websites in its search pages. The "summary" (technically called the "meta-description") will be displayed under each Google search result (or any other search results such as Yahoo, if you are using Yahoo Search). It is usually between 150-160 characters but I don't know why HubPages decides to use 200. Different search engines allow different length and I suppose HubPages choose the longest so that search engines would just cut off the 'extra' words.
I am not too sure whether you need to overemphasize keywords in the "summary" because there is another feature called "meta-keywords", which is still used by some search engines. Google, however, has decided not to use it because of too much abuse. Most important is that for the summary, you write 200 characters that will generate enough interest for people to want to visit your hub when they see it in the search pages.
I believe the search engine ranking of your hub depends a lot on the relevance of your hub's content and your hub traffic, rather than on the summary. So it's a chicken-and-egg story... low traffic, low ranking!