It probably wouldn't be too hard for you to reach 450MHz, but beyond that I wouldn't count on too much.
As for how to set the speed of the chip, the most common approach would be to switch the motherboard jumpers around to a higher speed. But your BIOS setting probably just changes the FSB on your motherboard--which does work, but you might experience system failures that might not be attributed to your CPU and are harder to detect because of the number of things that are changed when you change the FSB. Just try chaning the clock multiplier for your chip and then try playing with your FSB.
-JGene