how much Power Supply (PSU) do i need?

Hi,

Recently, I upgraded my mobo, ram, and my processor. and i seem to have a problem... sometimes when i boot, my monitor won't show anything, but i know the cpu is working because the lights in front of it are flashing.

now i suspect that the power supply is the one causing this, maybe it can't supply enought power?

When I buy a new one, what wattage do i need? Here's my specs:

- ECS EliteGroup MoBo
- 1 GB RAM
- No video card (just the built-in)
- 1 DVD/CD Writer

How can I know the needed power?
SUggestions? thanks

Mean 400W. 400W brand (FPS, High Power, Chieftec, Seasonic, Zalman) will suit all PC's tastes.