Bah, I'm stupid. Since the beta is over, you might not even be able to install, since it updates as you go.
Sorry, man. You'll have to wait for the real release.
As for Steam having higher reqs than HL2... this is just a guess, but maybe because steam runs on top of HL2...? You could run HL2 single player with less resources than with the Steam wrapper...? Maybe?
Could you link to where it lists the reqs that are higher than HL2?