You don't build a system by picking out the GPU first. A price range for the build, any preferences you have, and main use/s for the system, are needed. Then, of course, we could move it to the general hardware forum.
With that said, generally speaking @ <$200 the 6870 will be the highest performance card. You can occasionally find the 560ti for ~$200 though and it's a bit faster overall (not all games, obviously). Generally it's more in line, price wise, with the 6950 1GB though.