Unless you already have a well established website within the search engine ranks, selecting keywords with a high number of searches should be avoided. These sorts of keywords will usually mean that there is a large number of competing pages, which in turn means it will be extremely difficult to achieve high ranks.
Due to a large number of competing pages for generic terms, optimisers should focus on selecting keywords that have a reasonable amount of searches and a minimal amount of competing pages. The key is to focus on a small list of keywords, say 20 to 30, and then once a strong presence has been made for those keywords the list can be expanded to include slightly more generic terms.
Having said that, selecting keywords with less search numbers doesnt necessarily result in less visitors to your website. Search engine users these days tend to refine their searches to more specific keywords since they are given a broad range of websites when they use the generic terms. For example, if you have people wanting to buy a car, most search engine users will enter the phrase buy car. Once they are given a wide range of websites to choose from, they will in most cases change their search to include a car model type. For those who have optimised for the more refined phrases, the more likely they are going to get an interested buyer.
For those of you who are looking for a good keyword tool, Wordtracker and Overture is a good place to start. To determine the number of competing pages for a keyword phrase, enter that phrase into a search engine and look at the number of pages indexed. As a rule of thumb, I always tend to select keyword phrases that have less than a million pages indexed in Google or Yahoo.
With all that in mind, provided you target selected keywords that have a reasonable amount of search numbers and a minimal amount of competing pages along with proper optimisation practices, you should be able to achieve a strong presences in search engines over a period of time.