My first thought is to ask for a little involvement fromthe grandparents end. Hallmark makes recordable story books, you could buy the book mail it to Gma have her read a story and record it, and send it back to the kiddos. How sweet is that!
I would also keep manilla envelops handy and once a month or sooner pop in what ever artwork won't fit on your fridge and mail it out.
As for the skype we had trouble getting the sound to work at the same time the video did. but im techno challenged. If you go that route, i would give the kids something to do or hold while you planned your chat times. Like if you set up joey's lego table and had him build while he chatted with gramps they would both probably enjoy it more than just sitting in a chair for five minutes. Or have the kids at the table older one coloring a picture that you will then mail out to grandparents and the other can be in the highchair eating cheerios or something. Hope i'm explaining that, plan somethign to keep them occupied and tell grandparents to plan on keeping it super short like one minute per childs age since that is about their attention spans anyways.
kudos to you for working to give extended family importance!!!