Okay, i've got a weird problem

 

I have a Dell Dimension 8300, recently i bought a new Radeon 9550 and it works fine untill i try to play a game, then it freezes, unfreezes, freezes again then goes out and gets deactivated by the catalyst A.I.

 

I don't know whats wrong, and does my computer even support this card, or is it defective? I'm at a loss here