Half-Life 2 currently yields poor DirectX 9 performance with FX cards - the jump from the FX 5200 (entry-level) to a 9600 XT (mid-range) card will give you a large performance gain, especially when bandwidth-eating features such as anti-aliasing and anisotropic filtering are enabled.
I find the ATI card to have better 2D quality as well, but I guess it depends on the manufacturer of the card.
Specs are listed here:
Key features are Smoothvision 2.1 (multi sample anti-aliasing), Truform 2.0 (higher order polygon tesselation by using N-Patches, and I believe NVIDIA cards do this with RT-patches); HyperZ IIII (memory bandwidth controller) and the Videoshader feature, which supposedly improves video playback quality
I generally don't pay too much attention to the specs; it's how the card works in the real world is what matters.