Yes you will see a huge difference as you crank up the details and resolution, not so much in the image quality itself but the x1800xt will allow you to run higher antialiasing with less framerate drop and you'll get SM3.0 in the games that offer it. Biggest difference is HDR, X800 can't do it and X1800XT can.
All you need to see to convince yourself its worth it is Tomshardware VGA chart
http://www23.tomshardware.com/graphi...2=313&chart=91
Select the X800XL and X1800XT and check out the colossal difference in FPS