You can adjust the FSB without changing the agp/pci clock. I think via kt266 and up mobo can do this already.
You can raise the procc's total speed if your OC your FSB rather than the raise the multiplier. OCing the FSB does not strain your proc so you need to have a good cooling for your NB. active cooling is advised.
Try to lower your multiplier a little bit and raise your FSB, just maintain the vcore. This would even be faster than the default clock speed. Have the mem and FSB in sync ratio(1:1).
Games and MS Office applications generally improve with the higher front side bus speed.