That all depends on what else are you powering? For instance, there would be a huge difference on whether you're running a single videocard versus SLI or Crossfire.
You might want to list your components (i.e. How much RAM? What is your GPU? How many hard drives do you have? DVD and other components? Etc.)
To be honest, 450W is kind'of low to start with. If you're overclocking to get better performance in games then I'll assume you've got a powerful videocard (which takes lots of power from your PSU). So, you'll probably have to upgrade your PSU...to what? That will depend on your other components. Let us know.
At a 4.2Ghz OC and a core voltage of 1.4 with power management disabled: Idle draws about 173w at 100% load about 227w. At stock speed and 1.225 core voltage it idles at about 153w and at 100% about 173w. The link below is the source of the data. Hope it is helpful. You can use a power calculator to check the load of your other components to see if you need to upgrade the PS.
If you are just overclocking the CPU (E8500, 65 watt TDP), you will need less than 20 watts more from your PSU.
When I was overclocking my Q6600 system, I was curious about what that would do to system power consumption.
Q6600 VID = 1.2625 volts. Core voltage at 3.6 GHz = 1.425 volts. Measured 12 volt current, stock BIOS settings = 8.5 amps (96 watts) into the motherboard power regulator. OC'd = 10 amps (120 watts) running at 100% load. CPU power consumption increased 25%.