Gamespy asked if Half Life 2 is going to be a game which really is going to be pushing system requirements, here is the answer which Doug Lombardi gave :
Doug Lombardi: I will sound arrogant as hell if I say yes! But I think that Valve tries very hard to support as far back with system as we humanly can, and in this case we're going back to a Pentium III 800 with 128 of RAM, but as get up to a Pentium 4 class you'll see better water and better effects. For folks who want the ultimate experience, they'll want the latest ATI card, and the fastest processor available from AMD or Intel.
It is a pretty interesting article if you want to learn some more about Half Life 2, you can find it here.