Actually, in the X1600XT vs 6800GS, the performance difference is because the X1600XT is the world's fastest 4 pipe card -- it can only render 4 pixels to the frame buffer at a time. They can be very pretty pixels, but you only get 4. Don't get me wrong, the 50% more memory bandwidth the 6800GS has doesn't hurt, but it's probably not the main bottleneck at lower resolutions/quality settings you'd use with an entry level card like the X1600XT.
22 Gb/sec bandwidth on the 7600 (also the 1600XT) is nothing to scoff at. It's nearly double what you get with a 6600GT, which is a fairly capable card. At 'lower' resolutions like 1280x1024, it should be plenty enough to turn up the eye candy. That bandwidth is comprable to 256 bit cards with slower clocked ram.
I've also heard rumors of 7600s with 256 bit ram. Duno what the prices on those will be, but I'm hoping to pick up a 7800GT for around $200 soon instead.
Wonder how long until X850XT hits $125 or lower as well -- ATI still has truckloads of cores.