There are tricks to managing your Steam library if you are running low on space or simply setting up something new, from tricking Steam by copying files manually or the new feature which allows you to move games from within Steam.  One other possible way to manage your time and bandwidth is to build yourself a small little webserver which caches any Steam game you have downloaded locally, so you can reinstall them without using up your bandwidth.  Those familiar with Riverbed appliances and the like will already be familiar with this process but many gamers may not be.  Ars Technica walks you through the build and teaches a bit about caching and basic webservers along the way; check it out you are not already well versed in setting up something similar.

"But there’s an alternative to having to re-download all your Steam games from the Internet: you can set up a local Steam caching server, so that once you download something, you’ve got it on your LAN instead of having to reach for it across the net and incur usage fees."

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