You should be able to go to a wal mart or best buy and get it right now if its still in stock. A few days ago, the street date was broken and then they decided to start selling it also.
But you can either get it in store or download it.
and DH..your computer will run HL2 perfectly fine. Mine is an AthlonXP 3200 512 ram and a 9800Pro and I'm putting it on High settings at least.
The computer that Valve used for the reviewers was running a P4 3.2ghz, 2 gigs of ram (a little excessive

) and a 9800Pro and the reviewers said the game ran perfect on high.