2x670 in SLI are similkar to 2x680. Assuming you play at 1080p.
I wouldnt do the swap.
they are similar @ 1080p resolution. At this resolution youy are basically CPU bound.
I would sell the 570 and keep the cash. From your rig, i cant see anything you can upgrade that wouldnt be a sidegrade. nice rig, btw.