I have a Sapphire Radeon HD 4870. Since it does not support DX11 i'm thinking of updating it to 5670(some entry level DirectX 11 Card & Not into serious gaming). So does 5670 perform better than 4870? I've seen couple of benchmarks and the old card outperforms the 5670.
It depends on the games that you are playing or hope to play. There are a few advantages of upgrading but you will lose performance over all. The 5670 is on par to being slightly faster than a normal 9800gt which is a lot slower than your 4870 but at the same time there is dx11 and much more energy efficient.
4870 for raw performance
5670 for low power, low heat, much better power efficiency, and more modern api support.
DX11 is underwhelming, especially on a lower end graphics card where you will be running at medium to low settings. Going from a 4870 to a 5670 is a pretty serious step down in graphics quality.
My rule of thumb is to only make upgrades that are, well, upgrades. So a 4870 to something like a 6850 at a minimum--though that's really not a huge performance upgrade--you're on a tech site where people are really hung up on the latest greatest and fastest but the truth is the 4870 is a pretty kick butt card for most games--if there's a specific title or resolution you're currently displeased with then check out some benchmarks to be certain that you get the performance upgrade you want (for me it was getting 60 fps+ on Battlefield Bad Company 2 that prompted me to retire my 4870 but that was the only title I played that the 4870 struggled with).
In terms of raw performance the HD 4870 is basically equal to the HD 5770 and the HD 6770. The HD 6770 is simply a rebranded HD 5770, but comes with the newer HDMI 1.4a connection which is meaningless for nearly everyone.
Of course, the HD 4870 lacks DX11 support. DX11 in my opinion does not add much eye candy from what I've seen so far. Yes, DX11 offers a little better graphics effects than DX10. But to me, the de facto standard is still DX9 which was a huge bump up from DX8 and previous DX versions. DX10 was a tiny step in evolution and DX11 is a small step.