X-Plane needs a GPU with a very good OpenGL support. I'm using the Nvidia GeForce 8800GT. But you can test the free demos before you buy the product. If you don't care about eye candy, you can set minimal graphic details.
X-Plane 10: http://www.x-plane.com/downloads/x-plane_10_demo/
X-Plane 9: http://www.x-plane.com/downloads/older/
More about X-Plane 10 and GPU:
Here you can find a X-Plane test with a laptop (no dedicated GPU):