Well in the internet i saw that MSI ATI Radeon 6770 1 gb(memory clock 4400 mhz) requires 400 watts psu but what will happen if i dont have that much psu watts... tell me
You misread, or the PSU requirement was poorly worded on the site you looked at. The card itself doesn't need 400w. A 400w PSU is recommended to support a whole system with a 6770 in it.