Some info on Steam. No matter which "install" method you chose (CD/DVD or Dload), you MUST have Steam to play Halflife, CS, TFC, HL2, CSS, etc. Online.
As mentioned above, you can run in off-line mode as long as your LAN connection is not active. Not the same as if your internet connection is down. Just unplug your LAN cable from your PC.
I own the CD install version of HL2. About the only thing I used it for was the original install. Ever since then, when I reload my pc, I copy my recent DVDR backup of my steamapps folder (which contains the 8GB of maps I have for hl,cs,tfc,hl2,css) to my \games\steam\steamapps folder, dload steam, run the install, point it to d:\games\steam and let it dload the updates for my registered, transferred games.
That method get my Valve games back up and running without having to do a FULL dload of all my steam based games.
Getting back on track, I guess what I'm saying is that other than having to wait for your games to dload, there's really no difference between a CD/retail install and a "steam dload" install. The keys are the same.
Just make sure that when you create your steam account NAME, you use something you will want to keep for the rest of your life. You CAN'T change the account name, only the email address attached to it. So, if you think you might not want your future wife to see your login ID of csteenywanker, don't set it now.