An RX 580 wouldn't be much faster than a GTX 970, so if that's what you have for a graphics card, you would definitely want something significantly faster than that.
As for the "max powerful graphics card for your rig", a 6th-gen Core i7 should still be fast enough to handle just about any graphics card on the market today reasonably well, limited only by your budget, and perhaps by the wattage of your PSU, which doesn't seem to be listed there. Do you know what kind of power supply you have? And as was said, knowing what model of monitor you have would be useful, since a very high-end card might be overkill on something like a 1080p, 60Hz screen. And of course, how much you are willing to spend.