There is no difference...they're both equal ratios. But they might be used in conjuction with something else, depending on the motherboard and BIOS, such as determining the PCI/AGP speeds. For instance the CPU bus would be 4, and the memory bus be 4, and if those are at 133MHz DDR then the AGP would be a 2 (66MHz) and PCI would be 1 (33MHz). But if the CPU and memory were at 166MHz, then they'd be a 5.