the NorthBridge(now the IHA on some newer motherboards) is an interfaces the CPU with RAM, AGP, PCI(and PCI Express). its basiclly a support chip that takes some of the workload of the CPU. clock doubling was for older computers that need to speed up the Bus speed to run faster which was required since the CPU had to interface directly to the RAM and I/Os(without the NorthBrigde support chip), which toke up proccessing time and bus speed.
