Using www.gpureview.com I looked up the 6800XT and the 7600GT. According to that sight, they both have the same memory bandwidth of 22.4 GB/s yet the 7600GT is clearly the superior card. The 7600GTs core (which is it's processor) is clocked at around 560Mhz while the 6800XTs is only around 325Mhz. Per www.gpureview.com
Even with faster GDDR3 memory (1000Mhz) and a 256-Bit bus, the slower core of the 6800XT cannot keep up with the faster core (560Mhz) and faster memory speeds (1400Mhz) of the 128-Bit 7600GT. Those high speeds are what give the 7600GT it's ability to compete with older 256-Bit cards like the X850Pro or an X850XT.