The X1600PRO runs slightly faster then the 6600, but slightly slower than the 6600GT. The X1600XT runs slightly faster than the 6600GT, and probably as fast or slightly slower then the 6800. The 6800GS runs faster than all of those cards. So I guess to sum everything up:
6600<X1600PRO<6600GT<X1600XT<6800<6800GS