The Linode you need all depends on how well you configure your applications. It's hard to say what will work without knowing much about your setup. With that said, you can always try the $20 package and see how that works for you. If you decide that's not working, upgrading is as easy as shutting the Linode down and clicking a few buttons. There's a small amount of down time involved in doing so.
We also offer a few guides on Pantheon/Mercury here:
http://library.linode.com/application-s ... t-mercury/
If you don't feel like setting it up yourself, you can also deploy with this "StackScript":
http://www.linode.com/stackscripts/view ... riptID=353