X850XT PE - are they even out? supposedly they offer onyl a few fps more than the X800XT PE though
id's say for half life the benchmarks along show that at the top end ATI are leading, in the mid section you have the 6600GT which is as good at HL2 as the 9800Pro but will also cream doom3 too, in the budget section the 5xxx just aren't made for HL2 really, i'd go for a used 9700Pro
overall it depends what you like at the top, both 6800U and X800XT PE with take any modern game at any res, in the midsection i think nVidia have the edge with the 6600GT, in the budget section if you want HL2 performance then ATI are the choice
If your buying strictly for HL2 go for an ATI, I have a 5900(Which was the best when I bought it) and it doesn't do Dx9 correctly with HL2 for a variety of reasons, none of which involve it not being a "good" Dx9 card. However an ATI is better for HL2 unless you get a newer FX, then they're pretty much on par with the newer Radeons. IT is prudent to note that the newer Fx's still outperform the Newer Radeons in OpenGL, though I still think thats mostly driver suppourt.
Or if u are like me and like the Nvidia cards go for the 128Mb XFX Geforce 4 Ti4200 Turbo Gamers Platinum Edition. Go here for info...http://www.beyond3d.com/reviews/xfx/4200/
This card plays HL2 on full detail and i average 75fps, Also Doom 3 plays around 44 fps with full detail. See my sig for specs
I would also point out that just because the GeForce FX range doesn't run HL2 that well, it certainly doesn't mean the 6xxx range is no good. It's bloody amazing! (See my comment earlier)