The HD 5570 has similar performance as the HD 4670. It also uses less power.
The HD 5570 also incorporates DX11, but DX11 is a little more demanding than DX10 and significantly more demanding than DX9. The HD 4670 was considered as a "higher performing mainstream video card" when it was released. Now it and the HD 5570 are considered to be good multi-media cards because the performance envelop of more powerful cards are simply that much greater.