XFX GT240 512MB DDR5 GRAPHICS CARD

That depends... what are your other components? Are you planning on cooling or overclocking or not?

Normally, u will need a minimum of 500W of decent brands, like Corsair, OCZ, Antec, Seasonic, SilverStone...
Just to be play safe..  

minimum 350w

please don't reply to posters if you have no clue what you are talking about

Or if they won't reasonably understand that you are joking.