The HD 5670 is power efficient and has DirectX 11. However, DX11 is somewhat demanding and the HD 5670 isn't exactly in the high performance category.
The HD 4830 is more powerful than the HD 5670, but uses more electricity than the HD 5670 (roughly 85w vs. 30w). It only supports up to DX10.1 though.
In terms of DX9 and Dx10 performance....
HD 5670 < HD 4770 <HD 4830 < HD 4850 ~ HD 5830
See following review of the HD 5670; it does not include benchmarks of the HD 4830, but it does include the HD 4770 and HD 4850.http://www.xbitlabs.com/articles/video/display/radeon-hd5670.html
If you want DX11, then the HD 5670 is the card to get. If you don't really care about DX11, then the HD 4830 is the card to get because you will get better performance.