City 17 had an interview with Gabe Newell from Valve. The interview is about Half Life 2 ofcourse. Here is a quote about the system requirements needed to run Half Life 2, in 1280*1024 with full details :
Storm: We know the minimum suggested speed is 700 mhz, What are the recommended system requirements?
Gabe Newell: If you want everything turned on, running 32-bit 1280x1024? Hmm. I'd think you would run about 40 FPS with a 9700 Pro and a P-IV 2 GHz.
Given the scalability of many different parts of the system, you have to add a quality dimension and target frame rate.
In another interview Doug Lombardi from Valve said that a Pentium III 800 with 128 of RAM would be the minimum requirements to run the game.