Is my pc good?

Plenty of CPU power (overclocked or not) and memory. Good power supply and motherboard. GPU is going to hold you back if you start turning up settings and increasing resolution.
Light reading => http://www.tomshardware.com/reviews/gaming-graphics-car...

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.

you have a top notch fancy mobo with a very nice cpu with an overkill of ram huge power supply with a low end graphics. poorly balanced build

hi
what if i get another graphics card and crossfire it?is it an good idea?

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.