The AGP is set to run at 66mhz at factory settings and higher AGP bus speeds leads to corruption in video settings. You want to run you AGP at 66 because non-66mhz settings can cause serious problems.
Before locked AGP/PCI buses, a divider was used to set the AGP/PCI bus speeds. Thus a FSB of 133 would use a 1/4 divider, giving 66mhz for AGP and 33 for PCI (PCI is half of AGP), at 166, 1/5 divider was set. As long as FSB was set to either 133 or 166, there were no problems, however overclocks of anything beyond either two reference points had the potential for AGP/PCI corruptions (i.e. FSB 150 or 180).