Question: I have a .biz website that ranks well in some of the major search engines for a few keywords, but does not rank as well as I would like for many other keywords. Should I consider switching to a .com domain name?
Answer: In the long run I think it is worth moving away from .biz if you are creating a real long-term business. The web was created to share information, and businesses are generally viewed less admirably than the individuals that work in them. As long as relevancy algorithms are based largely on links, then a .biz extension could hurt your exposure in most fields.
Most web companies that control large traffic flow have taught their userbase (and those who they trust to vote) that commercial = spam. To put this in perspective, some journalists write entire articles about businesses and then do not link to the businesses because they feel that doing so would be too promotional. In competitive fields sometimes only a few links separate a business that is getting 5% of the traffic or 30% of the traffic a search engine offers.
If you are starting your business on a .biz it doesn't matter if you have the best information in the world...the bias of .biz (and toward business in general) is going to hurt your exposure, making your business less efficient and increasing your business cost. The small businesses that are best sustainable on the web are those that function as businesses yet have the feel of non-commercial sites (and acquire links as though they are non-commercial sites).