Video card upgrade

[NoClue]

Hey guys. Please forgive my ignorance, but I'm not very knowledgeable when it comes to computers. I would like to upgrade my video card and I'm not sure which card to choose. I have an HP Pavilion Elite m9452p. I'm not exactly sure what specs you need, so please let me know if you need more info.

processor model: Intel Core 2 Quad Q8200
memory: 8 GB
processor speed: 2.33 GHz
memory speed : PC2-6400
memory type: DDR2 SDRAM
video card: GeForce GTS 250
power suppy: Cooler Master 550w

Any info you can provide will be very much appreciated.

Thanks!