Rams!! add more rams (from 8gb to 32gb) depend on your money, and if you ask me at least 16gb of rams for serious development .
for the graphic card a gtx 960/760 or similar performing cards from ATI will run decently , a grx 970 is even better and the best choice if you you have the money , and if you want to spend more of course a 980ti or Titan x are even better. .
I think it totally depends on what your are going to do. If you want to create a minimalistic game you can work on a Laptop (I used my Surface Pro [3rd Gen i5, 4GB RAM] for presentations and it works fine). But if you want to build some next gen AAA stuff you need to have a system which can handle a lot more.
As fennec_sky already said, you need RAM. 8gb is enough to run most of the stuff but I would recommend to have at least 16gb.
Definitely more than 4GB, I don’t know how people can deal with less than 16 (I am using 8GB right now, all I have open is UE4, and Google Chrome) As for the video card, GTX 960 2GB will work fine unless you are making a large game (I have the GTX 960), but a GTX 970 would be even better.