The difference is the FSB. In the KT133 you run a standard 100mhz FSB, (x2 = 200mhz rising and falling clock cycles), these can normally only be overclocked to around 110mhz but you may be able to get higher. The KT133A is a standard 133mhz FSB, (x2 = 266mhz), which overclocks much better. I have seen people in the 160mhz's range. As far as changing the NorthBridge chip, I do not think you can do that although I have not looked at a MSI board. If you could I think someone would have said something by now. Let us know though.