Both a GTX560Ti and a HD 6950 are almost neck and neck in terms of performance. Since you've got a 18.5 inch monitor, 1GB GTX560Ti should suffice well enough. The Radeon card's 2gig VRAM will come handy only with higher resolutions.
You could take a look at a HD 7850 too.
I have decided to go with 560 ti. but which one should I go? evga / asus/ zotac?? clock speed is showing different for all of these, so I am getting confused!!