The obvious choice that comes to mind is the GTX 750 Ti. But it would be nice if you could give a more detailed description of your components. Some pre-builts like HPs have trouble with cards that are not on their support list.
What Phenom exactly, Phenom or Phenom II? Phenom 9600 Agena, Phenom X4 9600, Phenom X4 9650?
Motherboard make/model?
Power supply make/model?