Does anyone know what type of difference there is between the amount of monthly searches done for a keyword phrase with overture/yahoo (keyword tool) compared to google searches for the same month and same keyword phrase?
Hard to judge. If you assume that Google has roughly 2x Yahoo's market share, you can just double overture's numbers. However, the demographics of search engines vary pretty widely. Google gets most of the webmaster/techie/internet-saavy users, so a user searching for "vbulletin drupal integration" is probably using Google.