Is there any way I can get a CD or DVD of HL2 if I buy the steam versions? If not, will Steam allow me to download DOD:Source if I purchase the collectors edition of HL2?
If you buy the retail CE you'll probably have to buy DoD:S from Steam. If you get the Silver or Gold Steam package you get it for free. And no, you will not get physical CDs from Valve.
Is there any way I can get a CD or DVD of HL2 if I buy the steam versions? If not, will Steam allow me to download DOD:Source if I purchase the collectors edition of HL2?
You can only burn the files downloaded to a CD/DVD but if you want to install them again, it's just a matter of copying them in the correct folder on a computer WITH steam, AND you have to log into your account to be able to play the game, as the cd key is tied to your account.