I suggest you get the Orange Box since it comes with Half Life 2 and the two episodes, along with Portal for only $20. If you like Portal then you can decide to get Portal 2, which I believe is a masterpiece. You don't need to necessarily play the original Half Life games before Half Life 2 as the story hardly makes much more sense.
That's one of the reasons that Sierra nearly brought Half-Life to Dreamcast, and why it carried through with Half-Life on PlayStation 2. Sierra's Half-Life for PS2 is indeed a game worth owning
Now Update is out of date and the current build of Half-Life 2 has surpassed it. Cupa filled you in, but I just want to needlessly add that the best version of Half-Life 2 you can play is the VR mod that has an official store page. It was easily the best VR game of last year and feels like a native VR game. All current Vavle games (Half Life 2 [and its episodes], Portal, TF2, Counter Strike: Source, etc) run on the Source engine. When Valve announced they were rereleasing Half Life 1 on the Source engine, everyone thought they were remaking Half Life 1 with Half Life 2 graphics and physics. Only really worth a play if you are itching for some more Half Life action. It is a cut segment from the full game Half Life 2 which was just used for a showcase of HDR being implemented onto the Source engine. You can happily skip playing this if you so desire but it is a fun, little 'walk in the park' mission. aBgy.