P3n :
Will it help if I upgrade the cpu to 6600? Or the improvement will be very marginal?
CPU won't make as much difference as GPU. With everything maxed @1440p I get 86 FPS average with 67FPS min and at 1080p I get 106FPS average with 88 min. And that's with SLI 980Tis. For 1080p you're going to need a 980Ti to maintain 60FPS+ with maxed settings http://www.techspot.com/review/1006-the-witcher-3-benchmarks/page4.html; you can see there a GTX 980 can't do it.
Also see in the same review how they only got 4FPS difference by going from an i5 3470 @3.2Ghz to an i7 4790K @4.0 Ghz. If you're determined to get 60FPS maxed @1080p with a single card then I'd say get a 980Ti. If not then SLI 970s might do it. Upgrading your CPU definitely won't do it since yours is fine.