8GB of DRAM is better than 6, *GB is basically the min you want for gaming. Whether DDR3 or DDR4 will depend on the CPU, the new series of Skylake CPUs will primarily be running DDR4 Haswell and earlier will use DDr3
I am confused between AMD PROCESSOR AND CORE i5 and i7 processor. I am willing to buy a laptop woth rs 40000 ? please suggest
That is under $600 US, this would be on the low end of a system for gaming, will run new games on lower settings, and future games even lower.
You are probably going to be limited on Acer models, look for one with a 940m video card, or a 950m, but a 950m would be at the top of your budget if not a bit over.
Remember what you buy now is what you have, no upgrading. If you buy a laptop now that will barely run games that are out now, it will not run new games well. Spending about $7-800 now on a system with a 960m video card would be better than getting a low end one for games. Unless you are talking about playing games like LOL and SCGO which most people seem to be interested in now.