I believe the 6600 outperforms in some categories, doesn't it?
"As we have seen from our tests, the X700 XT packs a lot of punch into a small package. Most of the time its not quite enough to keep up with the NVIDIA 6600 GT, but the X700 XT proves its worth in the Source Engine Video Stress Test, and FarCry, and Unreal Tournament 2004. Most of the X700 XT's power shines through when anisotropic filtering and antialiasing are disabled (the exception being UT2K4). Current and previous generation OpenGL titles do show the Radeon X700 XT lagging the NVIDIA GeForce 6600 GT. NVIDIA has traditionally been stronger in OpenGL performance than ATI, so this is not really a surprise."
From AT Review.