I recently tried the Doom open beta and while my rig (6GB RAM) was capable of running it on max detail with good framerates, it would crash after a few seconds with a message about not enough system memory. Only by scaling down some settings could I lower my RAM usage enough to play without crashing.
That's about as current a game as you can get, so I completely agree with SkyNetRising. 8GB should be your minimum. 12GB should help with games that are even more expansive than Doom and 16 will give you some future-proofing. I think the one thing you don't have to worry about as much, is RAM speed since anything from DDR3 and up is pretty fast in any frequency. An exception would be systems that really depend on RAM speed, like a PC with an AMD APU and no graphics card.