I've built a pc and i want to see if its good for gaming.
mother board: MAXIMUS IV EXTREME-Z
cpu: intel core i7 2600k
graphics card: asus hd 6770
memory: corsair 16g 1600 (4*4)
psu: xfx 750w pro
It's mediocre. Twice as much memory as you need (and no, having twice as much as you need does not translate in any way into better performance), and the CPU is also much more than is needed for gaming. If you'd spent less on those parts and more on the video card, you'd have gotten much better gaming results.
It will definitely be a large performance increase. I'm not sure what the relative performance of Crossfire 6770s is. I'd guess probably somewhere in the range of a 6950/6970. Your PSU is definitely enough to handle the load.