HL 2 needs like half the power to run. I don't know how Half-Life did it, but it will run on some of the older comps. My comp is a P4 1.6, Geforce 4 MX, 512 DDR and I'm pretty sure it'll run smooth. Maybe not maximum, but I can't run most of the newer games at maximum without lag. Doom 3 on the other hand needs a lot of power to pack. I got a ripped Doom III version demo (cough*Don't ask me how or where*cough) and it was freaking laggish on my computer. Although I think it had a little to do with being a demo, but I could definitely tell my ocmp was nothing for it.
About HL 2 and Doom III on X-Box. They will look great, but not nearly as great as their PC counterparts. Just like Star wars KOTR, they are beefing up the graphics for the PC version. they'll just lower the power needed for the X-box versions.