I want to be able to play Civilization 5 on the release date, but I know that it will be at least a 4-5 GB download, so I worry that if I buy a digital copy, the high number of users hitting the distributor's servers will slow downloads to a crawl. If it's possible, I'd like to download the game in advance, so that on release day, I would only be getting a tiny activation key instead of a huge installer.
The game is available for preorder on both Steam and Direct2Drive, but neither product page seems to mention what I'm looking for, and I don't know what it's called, so it's hard to search for. Do either of them offer this service?