My advice is to go with 3 to 5 words searches to start when you are starting your SEO program. If you have an older URL, try for 3 and 4 word searches. I go for between 300 and 500 words per page. Don't go crazy putting alot on the homepage it can only do some much. Use content based text links to link to other optimized pages with in the website, DO NOT stuff your homepage.

I would use the Overture data to get an IDEA of good searches but remember, these numbers show traffic not purchases. Thats why niche is so good. These searches are at the level of specificity that they are buying not just searching.

Example - Portland Oregon real estate - the numbers say that 10's of thousands search that on Yahoo. OK, sounds good. why do I not want to just optimize for that search or put all my apples in that basket. Well the fact is that endusers are searching that string but using it to gain more detail for another search. They are checking neighborhoods, prices areas and then taking that information re-searching with more speciifity based on areas or services.

Making your site come up for all the neighborhoods in Portland is far more advantages then one big search. Also search recognition will get you search revognition. Endusers are seeing you more than your competition. In otherwords getting up for Niche searches will help you get into the bigger more competitve searches and knock someone out that has been intrenched there for awhile.

Good Luck