Nvidia’s GeForce 9800 GT is the winner in the mainstream category. It is far faster than its opponent Radeon HD 4830. The latter provides an acceptable frame rate at resolutions up to 1680x1050 inclusive, but has no reserve of speed at all.
The same goes for the Radeon HD 4670 and GeForce 9600 GT, except that these bottom-mainstream products cannot provide comfortable conditions for play at resolutions higher than 1280x1024 unless you turn full-screen antialiasing off.
Summing it up, Nvidia’s solutions are the best choice in the mainstream sector. And you should only buy a GeForce 9600 GT if you want to save some money and do not have a monitor capable of working at resolutions higher than 1280x1024 pixels.