Yeah, you will be dissapointed if you get the X800XL, because it lacks Shader Model 3, which is required to take advantage of all the new advanced DX9 features. Without it, you will have to play newer games in DX8.1 mode, which doesn't look bad, its just not as good. Get the 6800 GT, you won't regret it.
P.S. I have an X800 Pro...don't make the same mistake I did.