the pci divider is a ratio to figure out what frequency the pci bus will run at
there are 3 mainstream pci ratios for amd motherboards:
-at 100mhz fsb for cpu, ratio is 1:3 so pci is running at 33mhz
-at 133mhz fsb for cpu, ratio is 1:4 so pci is still running at 33mhz
-at 166mhz fsb for cpu, ratio is 1:5 so pci is still running at 33mhz
those are automatic on most boards, some boards have manual ratios you can set up, but most are auto
the new nforce2 boards allow the complete locking of the pci bus(and agp bus) so there are no more ratios altogether
they are there to keep the pci bus in spec (33mhz)
it controls many things: pci cards, ide controller, onboard resources all run off pci bus