HD6870 + GTX580 or GTX680?

Please help me make a choice for realtime 1080p monitoring 2.5k dng from BMCC.
Now my primary video card HD6870 and I want to add GTX580 (3Gb) or GTX680 (4Gb).
What of it better for maximum CUDA performance for Resolve?
I read that GTX580 is better than GTX680 for it.
And it makes sense to replace the HD6870 to another?
I tried HD6870+GTX570 (1.28Gb) - sometimes working normal, but sometimes slows...
maybe because video memory not 3Gb?
And my PSU has 750W - enough power?

PS System - hack.

Thanks!