The 6950 is the alt. to the 560ti, but around the same price. I'd suggest the 6870 over the 560. It's slightly faster, but slightly cheaper. Unless you really need PhysX and CUDA. Also, get 4GB of RAM. For gaming, 4GB is the standard.
Well, PhysX adds dust/particle effects, but few games implement it. WoW doesn't.
Here's a pic:
And CUDA? If you're using Maya, Premiere, and video editing/graphic design software, it speeds it up considerably. It doesn't add to games. AMD uses a CUDA alt. called OpenCL, which is an open standard, but gets less support.