A WP installation isn't only to install plugins but need care about configuration, secureness and many other things, that if you have one blog, isn't a problem but if you are a web agency and recursively need to install/configure/finetune WP, this is not the right way.
The fastest way, IMO, is to maintain a base installation, configured, updated with free and premium plugins (those that are used in all installation) all finetuned and simply clone that one when create a new site.
There are many backup/cloning plugins, both free or premium, out there and all of them allows to clone a full functional site that need only to customize for the new site, adding specific plugins and may be theme.
My 2 eurocents

Mac

P.S. Sarah, you are always the best