Seriously, absolutely the best solution out there right now for doing what you are asking and its free. One folder that can be shared across Macs, Windows, Android, iPad/iPhone, etc. Can't be beat and I've tried them all.
Definitely the Dropbox as well. It's the easiest way to keep your files in sync, and then access them as needed on the iPad. I have 4gb from Dropbox and 50gb from Box.net, but the Dropbox is 10x more valuable to me because it's supported by so many apps.
One more little hidden feature of Dropbox you may enjoy, particularly when editing pictures on-the-fly.
Dropbox supports a "time machine" sort of file backup. If you store an original file in your Dropbox, then edit it and re-save it using the same file name as the original, in most cases you will forever lose your original. In Dropbox, you can use their website to go back and selectively restore older versions of the same file. I've used this a couple of times and its saved my bacon when I accidentally saved a large Excel model, overwriting the prior version (that turned out to have the correct data).