Diablo – ny way to buy the Starcraft 2: Heart of the Swarm digital deluxe edition

Campaign progress is stored online. Swapping computers (or reinstalling) shouldn't affect that.

Now that I think about it, I'm not sure if you can save a game in the middle of a mission like you could in SC1, and if so, I'm not sure if that would be saved. But completed missions are completed on other computers.

Update: I have noticed some things that just don't get saved. Like if you finish a mission, then just purchase armory upgrades, then quit. It doesn't always save the armory upgrades. When the campaign is continued, it picks back up immediately after finishing the previous level. But this happens even on the same computer, and isn't any worse if you reinstall StarCraft II

Blizzard on Starcraft 2 Save Games: Starcraft 2 Blog

You can pre-download Diablo 3 once you've purchased it online. As @dlanod mentioned, you can generally pre-download Blizzard games.

However, to more directly address the question you have proposed you need to remember/be-aware that Blizzards downloader is a Peer to Peer downloading scheme. They use small feeders to populate different sections of the game to different people and then all of those people are offered up as hosts to everyone else to get those parts.

So the situation is not really a few hundred thousand people all hitting one or two servers for the game, but a few hundred thous and people all getting a small piece pretty quickly from the server like normal web browsing or MMO traffic or whatever, and then sharing that with everyone else as long as they are downloading.

There should not really be any release day concerns with access the server to download the client to your system.