If you are getting the punch you should definitely check out the Borders & Corners stamp set -
http://www.papertreyink.com/stamps_bcsquare.html. It is made to coordinate with the square punches. You could also click "idea gallery" for some ideas.
On most of the stamp sets she lists the dimensions so you can get an idea of the size of the dimensions. If you have not been to Nichole's blog, you should check it out for some inspiration and ideas (
http://nicholeheady.typepad.com)
I think a lot of people use the nestabilities these days. You might want to consider that if you have a die-cutting machine.
PTI is great - prepare to be hooked. Make sure you get some paper too!
Aimee