ATI Radeon HD4650 on a Dell XPS410

Hello,

Playing Modern Warfare2 on a Dell XPS410 with a Nvidia 7300le card, and I am getting Blue lines on the screen and yellow marks all over the place.

Bestbuy employee suggested new card, sold me ATI Radeon HD4650 1gb for $79. It says it needs a 400watt power supply but the dell only has a 375watt....

Will this card work, or should I get a different card? I really don't want to upgarde the power supply or spend a ton of money as this is the only game I play, and it is not very often that I do play.

Any help is appreciated.