There are literally dozens of SKUs (Stock Keeping Unit—think of it like a very specific model number) of graphics cards on the market today. Many of them are simply holdover stock from previous-generation products. There is no reason for Nvidia to continue to produce the GeForce 6600 series chips when the GeForce 7600 costs them the same, costs card makers the same, and performs better, right? But you can still find a lot of those cards out there.
One of the rather interesting graphs they published is a Dollar Per FPS comparison. You can check it out over here.