"NVIDIA’s NV3x line of GPUs – they still must be treated as DirectX 8 hardware, otherwise they suffer extreme performance penalties when running Half Life 2 using the DirectX 9 codepath. To give you a little preview of what is to come, in DirectX 9 mode, the GeForce 5900 Ultra offers about 1/3 of the performance of the slowest card in this test. If you’re unfortunate enough to have purchased a NV3x based graphics card, you’re out of luck with running Half Life 2 using the DX9 codepath (at any reasonable frame rates)."
The Half Life 2 GPU Roundup Part 1 reveals the oh-so unsurprising reality that to get the best from HL2, you need to buy ATI. So the other 50% of you (statistics made up) need to start saving.
" For Half Life 2, it does not seem that the extra price of the GeForce 6800 is worth it - the 6600GT does a fine job on its own and performs just as well for much less."