It depends on the chip. Some chips take to being overclocked better or worse than others due to variances in production. You also have to remember that any 040 chip you find is going to be 15+ years old, and the electrical pathways break down over time. It's a crapshoot, really. Sometimes you can use the stepping number to tell you if a particular run is more or less prone to overclocking based on reports from other overclockers using that version, but not always. If you are going to be putting a 25mhz chip into a 33mhz socket, then plan on digging up a heatsink, a tube of thermal compound, and possibly even a cooling fan.