since 4x AGP are found on older boards like 845 chipset, the performance of cos will be slower than the newer 865/875 chipset with hyperthreading, dual channel DDR....etc
but gaming performance hightly depends on the graphic card, since 6600GT is not really top of the end card, it will not make any different whether it is AGP4x or AGP8x
it is quite difficult to test if there is any performance drops if you compare AGP4x on 845 vs AGP8x on 865. FYI, i am running a X800XT PE, which is the fastest card u can get from ATI, on a 845 board. i can't tell u if the bottleneck is the AGP4x slot...may be when i am free then i plug it to a 865 board..and then force the board to run in 4x and 8x mode and compare the different.