The only option is to adjust the bus speed. Adjusting your bus speed get's sticky because you're also overclocking your ISA, PCI and AGP buses. The bonus is, if your cards can operate at the new higher bus speed, your whole system is running faster and will perform better. Another thing to keep in mind is that your memory might also not be able to handle the higher bus speed. Good, name brand memory usually overclocks better than generic memory.
Good memory is a little more expensive than generic memory, but it really is better.
Even if your system doesn't use a 133Mhz bus right now, you probably will upgrade to that in the future and you'll already have the memory for it. Also, if you're overclocking a 66Mhz or 100Mhz bus system, you'll get some breathing room with your memory. This is a sure way to fry your CPU if you increase the voltage too much. Use the very smallest increments and stop when your system is stable. Your CPU will run hotter than it usually does when you overclock it.