Is it possible to change the graphics card on an sl510 lenovo thinkpad?

I have a lenovo thinkpad sl510 laptop. I want to run big games on my computer like fallout new vegas and call of duty, and all the parts of my computer (processor, ram, etc) are fine for running them but my video card is obsolete. It's a mobile intel (r) 4 series express chipset family. I really want to change it to a ATI Radeon HD 4850 or something powerful but I heard that you can't change the graphics cards on laptops... Can someone please tell me if my sl510's graphics card can be changed? Thank you