Re: HL2 Backup Question
There is no need to back up Steam files. Anything you purchase and have a lisence to use over Steam will automatically allow you to redownload it in the event that your harddrive crashes, you get a new computer, etc. For example I could go over to my friend's house right now and use my account on his computer, and I would have access to every Steam game I own. In fact, I could log onto a dozen computers and download the games a dozen times if I wanted to, afaik there's no limit to downloading Steam files once you have a lisence for it.
But, all of the .gfd files located in the Steam/SteamApps folder also contain the data for all the Steam games (and there are also a few other folders for things like mods, but you don't really have to worry about those). Just backup the ones that you don't want to download again (they're named Half-life.gfd, Counterstrike.gfd, etc), and then copy them back if you need to make a new Steam install.