1. When I got a new machine, I simply installed Impulse on the new one, logged in, and could redownload and install the game without trouble. I think as long as it's just you using it (ie you're not playing it at the same time on both computers) you should be fine.
2. If you buy it through Impulse what happens is the digital version is registered as soon as the order goes through (due to the early release I"m not sure if that's today or tomorrow for new orders, but original release was supposed to be tomorrow). You can download and install the digital version immediately after the order goes through, without waiting for the box.
If you don't care about having a boxed copy, save yourself the shipping and just get the digital version.