anallawalla - 10:59 pm on Mar 28, 2009 (gmt 0) Work on deep links to each topic from other authority sites for those topics.
Don't look at the website being broad - look at each topic and make sure that each topic has its main keyphrases optimised. Wikipedia is a good example - zillions of topics, yet most generic searches bring a SERP with Wikipedia towards the top of the page. If you owned Wikipedia you would not bother with keyphrases such as "[something] encyclopaedia". You would simply design a great set of page templates and allocate some rules for tagging headings, inner links, alt tags etc.
Work on deep links to each topic from other authority sites for those topics.