which mother board is the best and hw much ram should i use for best gaming pc?
Motherboard depends on what make of CPU you want to get (Intel or AMD) and whether you want to overclock or not. That aside, one that can handle a decent graphics card for gaming, so it should have a PCI-e 2.0 16x port ideally.
You should be fine with 4GB of RAM. Corsair Vengeance or G.Skill Ripjaws are the brands you want to keep an eye out for.
For gaming, you really ought to be concerned with the graphics card and CPU. When building a system, decide on CPU first...then the motherboard comes from that. GPU is down to what you can afford and what's best performing at the time (and if you have a preference for AMD or Nvidia).