While that may just look like a little darker image in a still pic it will ahve a HUGE overall effect on ingame play. The exact same will be True for HL2.
Sorry Hellbinder, but you're wrong. Graphics have nothing todo with gameplay. Alot of good Quake3 players, turn graphics down to minimum for far faster frame-rates, and get much more enjoyment out of it.
I remember ppl playing HL1 in software mode and not caring even tho their card could do it in hardware.
Seriously, if you step back from inspecting every pixel of those comparison shots, the difference is pretty negligable unless
you're comparing two identical frames together like that, which 99% of nvidia card owners simply wont be doing!! Harcore graphics enthusiasts like you make up a tiny fraction of the game playing public.
And I have to say the lighting may be mucked around with, but unless you show every nvidia card owner the same image on an ATI, they wont even notice.
And I agree with someone else's post, I think the texture detail is better on the NV card. The window canopoy on the vehicle on the top right, is far blurrier on the ATI card. Some of the hills look more crisper on the NV card to.