A Q&A with Gabe Newell from Valve, hints on how current 3d cards will perform on HL2 and some slideshow shots from ATI's "Shader Days" event. Expect even more to come in the next days/weeks however in the meantime there is already a bunch of information out of the door for you to check out.
First of all, Valve seems to be extremely proud of their DirectX9 implementation with Half-Life 2 & Microsoft is not unhappy about it either. Now what you wanted to know, although online press benchmarks are not expected until tomorrow, Valve showed off some performance graphs at the aforementioned event... NVIDIA will sure get hurt (this is the kind of bad press no one will ever appreciate) and the fact is, GeForceFX boards are put to shame in HL2 tests, once again Valve assures us the game has a 'pure' DX9 implementation and should represent actual performance of the platform.
That shot (taken from Tech-Report article) shows how boards performed during a normal DX9 test (no special code), Radeon boards totally outperform even NVIDIA's most expensive product.
On a later slide, Valve shows how NVIDIA boards' performance is improved by using a special codepath for the NV3x chips (which they say took a lot of time to build): 5200s and 5600s boards get some ~15% improvement which will probably do little for actual gameplay, while high-end 5900 FX takes a ~70% boost, matching mid-range 9600 Pro performance.
There seems to be little hope for FX boards in next-generation apps, new Dets will most likely help but not enough to change the whole picture... how will the graphics giant manage this? I would say Doom III could be their only hope in order to regain some of gamers' trust, in any case, could two different well-optimized DX9-based titles differ so much (performance-wise) that could affect NVIDIA positively?