I am on tight budget. So I am going to get A10 5800k , 4 GB DDR3 RAM with a discrete Radeon HD 6670 1GB DDR5....will my system able to play modern games like Modern Warefare 3, Dirt 3, BF 3 , Tomb Raider 2013, Sleeping Dogs, Crysis 3 in 1360 x 768 monitor....in medium to high settings....I am not a hardcore gamer....so medium settings on most games will do for me.... Will this games must need the dual graphics or i can cut the gpu from the bugdget.....and how much increase can i see by using HD6670 as a dual graphics partner for A10??
Check benchmarks for the Radeon 6670 for those games. The Dual graphics does not benefit the 6670 much, and very few games will have a noticeable performance gain over just running a 6670 by itself. For those games, you are probably looking at low to medium settings, not medium to high. However, the 6670 is about twice as powerful as the A10 APU alone.
I would drop to a cheaper, non APU, Quad core (Phenom II X4 or FX4000 series) and run a 7770 at minimum, and a GTX650 Ti if you can find one one sale. You'll be a lot happier with the performance.
I know you are on a budget, but this is one of those things where each increment is HUGE. Going from a 6670 to 7770 for $30-40 is like night and day and then same with the jump from 7770 and 7850. But after the 7850, you need to spend like $150 more to see those kinds of gains again. It makes the difference between struggling to get 30fps, to just running smooth as glass.
My "budget" recommendation would be to run an FX-6300 and a 7850. Maybe only cost you $100 more than you'd planned for literally several times the game performance.
I have a A10-5800k with 6670 g card its really good! not at 1080p though but at 720p I get about 70-80 fps on bipshock infinite! But I bought a new g card and its a HIS 7870 and its way better then what I had before!