I currently have a 9800 Pro and I am personally not concerned about its ability to run this level at all whatsoever. Half Life 2 and the Source Engine is not a very GPU demanding engine considering what's availible on the market right now, and it would be very bizzare for its requirements to skyrocket to 16 pipeline cards.
Now, that's not to say that you get away with 1600x1200 with 4x AA and 16x AF on a 6600GT or anything similar, but I'm positive it'll be playable with fair quality on a 6600GT pretty easily.
Intel C2D E6320 / AMD Athlon X2 3800+
Gigabyte 965P DS3 / DFI nF4 Ultra-D
2GB OCZ Gold PC2-6400 / 2GB OCZ Gold PC4000
eVGA 8800GTS 320MB / eVGA 6800GS 256MB
150GB Raptor / 74GB Raptor
2x500GB / 320GB
OCZ GameXStreme 850w / OCZ StealthXStream 600w