Computer picture gets blurry, barely functioning. Graphic card problem?

Hello! I do not know where i am supposed to post this. Please redirect it if i am to wrong

I have some troubles with my old computer. It’s a dell Dimension 8400. Recently I have had some issues with the picture. It get a lot of stripes and then it almost stop functioning. I also get a error message from the graphic card:

The Nividia system sentinel is reporting that the NVIDIA powered graphic card is not receiving sufficient power. To protect your hardware from potential damage or causing a potential system lock u, the graphics processor has lowered it’s performance to a level that allows continued safe operations.

The message also contains some kind of solution: That the problem is the supplement power connector, it needs to be attached.

The graphic card is a NVIDIA GEFORCE 6800 GTO. I have not changed the card, it came with the computer.

 I would like some more information or tips have I can find a solution to the problem. The power supply is 350 W and the graphic card need less. And like I said, the computer is old so it have been working before.

What should I do? Change card? 

Kind regards, Ludvig Larsson.