There are some Magento Connect extensions that I find myself installing every time I create a new project, such as Fontis_Recaptcha, Mass_Product_Relater, HM_DeveloperToolba
I would take another approach. Install a clean installation and add all your extension. When your done export the DB and zip all the files. Then you should be able to use the files and the DB as a base for all your magento installations...
I do the same with Joomla, have not tried that with magento yet, but it should work.