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.