Looking to change my graphics card

Hello,

Im new to changing hardware in my pc and was hoping someone could tell me if I am able to just go out there and pick up one of the new graphics cards if I want improved performance on my games.

below is what i have:

Motherboard: ASRock Penryn1600SLI-110dB
CPU Type: QuadCore Intel Core 2 Quad Q6700, 1600 MHz (6 x 267)
Graphics card: NVIDIA GeForce GTX 550 Ti
Ram: 8GB

I am not sure if I can just go out and buy and PCI graphics card that I want as I am not sure what is compatible with my motherboard etc.

Any help greatly appreciated
Allan