3DMark06
I've already pretty much told you the story on 3DMark performance, but here's a complete set of results at the benchmark's default settings.

The 2600 XT is neck-and-neck with the 8600 GT across the entire range of resolutions. The 2600 Pro, meanwhile, is clearly more capable than the GeForce 8500 GT, which performs almost identically to the Radeon HD 2400 XT.

As I've mentioned, the picture changes if we enable 4X antialiasing. Then, the 2600 XT drops behind the 8600 GT and the 2400 XT falls below the 8500 GT. The 2600 Pro, however, remains well ahead of the 8500 GT.