Via Eurogamer via Neowin.net:

But for those who want to see the game running in all its glory, be prepared to spec up to around a 2.4 GHz machine, with 512MB RAM, Windows 2000/XP and a DirectX 9 capable graphics card.

2.4 GHz machine - check
512MB RAM - check
Windows 2000/XP - Duh!
DirectX 9 capable graphics card - check

Looks like I don't have to upgrade (again) for Half Life 2.

Man, could you believe I actually posted a blog entry on the specifications of a computer game? But then again, we're talking about Half Life 2 here guys.