Almost any GPU can deliver framerates of 50+fps.
Trick is getting a high framerate at a specific resolution and the eye candy turned on. Multi-1080p monitors at maximum settings - a 7870 doesn't have a chance then again neither a 6990 or 690 will max it out graphics settings at 5760x2160.
Though to actually answer the question - a 7870 is a pretty good balance in price/performance for 1080p gaming. That said I'm a big fan of Intel CPU's these days - especially in a performance system, but there are arguments for AMD CPU's as well.
Will 1 monitor at 720p get 50 fps on high or ultra settings