Half-Life2 surpasses Halo 2. Halo 2 is only good for multiplayer. And the supposed ragdolls in Halo 2 were HORRIBLE. Half-Life2 is better for singleplayer. I don't think it's very good to compare a PC and console game. FPS games always work out better on PCs, IMO.
I have the same specs as you. Except I have p4 2ghz. (I don't care if it's low or not.) And I just got a 6600gt 128mb card and I can run HL2 on high settings fine, the framerate will drop in a few places and that's it.
I bought HL2 retail. It seems like it would be faster to install it than...
I run fear at medium to high settings (only at 800x600 though) and the game runs fine. I love the demo, I played through the part just killing the two guys that don't know you are there about 20 times. I loved trying different things to kill them. The AI in F.E.A.R. is smarter than HL2 AI, IMO...