A) since you are buying from steam, no keys are directly involved; if the DVDs you have are not already registered on steam, you can go ahead and give those up I guess
You don’t get the games until the Orange Box is actually released (October 10th), at that time, there will be a little gift box next to the games you bought a multiple of, you can click the box and send the game to someone’s e-mail address, where they can add the game to their steam account.
So basically everything is based on steam accounts, not CDs/DVDs or keys or any of that nonsense.