Most of the newer cards such as the ones in the GeForce FX line are all DirectX9 compliant while the older GeForce 4 line are all DX8 compliant. There are quite a few games that are optimized to take full-advantage of the features in DX9. No, they don't 'require' DX9 to play, but combining a card that is able to take full advantage of DX9 would enhance the playing experience, hence, newer technology. Plus, there may be a time, not too far down the road, where a DX9 compliant card will be required.
