Its a question of how far your individual devices will go before they crap out on ya. There are plenty of PCI cards, video cards, and hard drives that can handle a 40mhz PCI speed and 80mhz AGP speed, but if you have on sensitive device that craps out at 35, then thats as far of an overclock as your systems gonna get.
To single out the problem, try overclocking with as little devices as possible in the system (just video card and hd) and see what the max overclock is with them. Then add in a card one by one and see if everything works. If so, then either your hd or video card or processor is limiting you.