RAM 8gb vs 4gb

Hello I am looking to build a busget gaming PC and I was thinking about not going for 8gb of RAM and getting a 7850 instead of a 7770.  I will use it mainly for gaming and school work.  Should I get 4gb and will it be bad not having 8?  BTW wats the difference?

for gameing 4gbs will do but 8gbs is the sweet spot (4gbs used for games is the max you can use)