You must use a PSU calculator and input the speific configuration of your computer and also those devices that you may add later (USB, additional cards, more RAM).
The info you have supplied is not enought to determin an exact answer to your question. If you asked, "I want to add a video card to my system and not sure if my PSU is appropriate. Where can I calculate wattage needed for additions to my computer?"
You would get an answer like this:
Here is a place to start ASUS Computer Inc PSU calculator
Enter you items, CPU type, mother board type, Ram,fans, USB/Firewire items and so on. It will calc the MIN required PSU Wattage. This will be the MINIMUM. So if after you input all you data and it comes to 450, you will need to be sure you add nothing else onto the system physiclly to exceed the wattage. In fact, it would be wise at that point to invest in a new power supply that will have more than you need.