The GeForce 6600 GT has 128MB of GDDR3 on a 128-bit memory bus, which runs at 1GHz providing 16GB/sec of bandwidth. To combat this, ATI is making the standard memory width 256-bits wide. ATI will give you the option to have either 128MB or 256MB of GDDR3 on the card. At the 980MHz frequency that is standard with the 256MB version, this gives the ATI Radeon X800 GT a whopping 31.3GB/second of memory bandwidth, which is almost double that of the GeForce 6600 GT.
This is what we meant by a configuration that seems somewhat imbalanced. You have a 256MB framebuffer with a whole lot of memory bandwidth available, but only around 3.8 GigaTexels a second of fill rate available to it. Therefore, it begs the question as to whether or not this is overkill for the shader and fill rate performance the video card provides.
The X800GTs have higher bandwidth, but the throughput is roughly the same. In fact, I did the math once when I was shopping for video cards, and I think the 6600GT actually has a higher fill rate. I think it was over 4 GigaTexels, but don't quote me on that.