how easy is it to do this:
use my existing 500gb drive for now for files and documents
and when a good deal pops up, i replace the 500gb with another drive.
Since windows wouldnt' be on it, is it simply remove the old drive and put the new drive in or do i have to reformat or something?
This is a no-brainer and easy to do. You can simply clone the old drive to a new, bigger drive when that time comes. I use clonezilla for tasks like this all of the time. Get it here:http://clonezilla.org/