The processor already has some on board graphics : Intel HD Graphics 2500
Not worth using, and certainly not capable of playing BF3 on medium. Not even capable of playing it on low, really.
Sapphire AMD/ATI Radeon HD 6670 1 GB DDR3 Graphics Card Rs. 4864http://www.flipkart.com/sapphire-a
Would it be able to play BF3 at medium settings.
Yes, and fairly smoothly too. I'd expect 30-40 FPS at your resolution on medium in BF3.
Also what is the difference between DDR5 (seen first time on flipkart) and DDR3
GDDR5 is faster, but it's not going to make a huge impact on low end cards at low resolutions. Get the GDDR5 version if you can, but if not, don't worry about it.