Considering that you will be playing games at 720p display resolution, I recommend you to buy the following parts to build your custom gaming PC:
Motherboard: MSI 760GM-P21 (FX) Motherboard is the best budget Mobo which supports AM3+ socket types and easily handles some high end processors and GPUs. Moreover, it also has automatic overclocking feature which sets up the desired clock for your processor. It comes with one graphic card slot and two RAM slots. And of course supports DDR3 versions. It is available for around Rs.3k.
Processor: Amd Fx 6300 3.5 GHz processor is the best processor which comes with 6 cores that can easily handle multi tasking and graphic demanding games. It has unlocked cores which means it can easily be overclocked. Regarding this in view, the motherboard which I suggested you above goes perfect with this processor as it can automatically overclock the processor to a desired rate.
Corsair 2x4 GB DDR3 RAM: I strongly recommend you to buy only Corsair DDR3 RAMs which come with good frequencies and are well known to support ultra HD gaming.
Sapphire ATI R7 250X graphic card: And for playing all the latest games at 720p resolutions, you will need this gaming graphic card to smoothly run all the latest graphic thirsty titles.
Corsair VS550 550 Watt PSU: A PSU stands for Power Supply Unit and is meant for proper supply of voltage to different parts of the CPU. A 550w PSU should suffice for the configuration mentioned above.