The 6670 has almost twice the performance of the integrated graphics. But if you are serious about gaming i would suggest even a better card then the 6670. Its hard to find benchmarks displaying intel 4000 graphics compared to a 6670. Because they are pretty far apart but take a look at some of these for reference.
If you are buying a processor and can't afford a discrete GPU, than don't buy a 3570k. You're are wasting money if you do that. Get a lower end i5 or an i3 for Intel. AMD APU may be a good way if you can't spend a lot of cash.
More info is needed. (What is the PC for? What parts do you have? Budget ? just to list a few)
What will be your graphics use?
For normal desktop activities like browsing, HD movie playback, and static strategy games, the HD4000 graphics of the 3570K is entirely adequate.
If you want to play fast action games like first person shooters, or will be using a 2560 x 1440 or larger resolution monitor, then you will want a discrete graphics card.
The 6670 qualifies, but just barely as a gaming card.
For gaming, your 3570K deserves a card in the GTX670/ 7950 class to be balanced.
The Radeon HD 6670 much more than 2x faster than an Intel HD 4000 graphics core. Not even the upcoming Intel HD 5200 (which is supposedly has twice the performance of the Intel HD 4000) will come close to the Radeon HD 6670's performance.