Hello there, mate
Well, the HD 4890 is similar in performance to a GTX 280, it's better than a HD 4870 of course but is less powerful than a HD 5830, GTX 285 and GTX 460.
You'd need it if you run games at extremely high resolutions with extremely high details on graphics, or if you're an enthusiast
The HD 6900 Series are actually top on the line, HD 6970 is between the GTX 480 and the GTX 570.
But again, if your games are perfect now, you don't need to upgrade
Also, HD 4000 Series supports Dx10.1, HD 5000/6000 series have full Dx11 support, that's another thing to have in mind.