faster or bigger ram you ask? depends...
if you're talking about gaming, and your graphics depend on an integrated chip, like an AMD APU or simply the standard "HD" graphics embedded to core i intel processors, then your ram SPEED will have a huge impact on grapfics(fps) so if you go from single channel to dual channel memory for example, you will see an enormous improvement most of the time, graphically speaking, since that kind of graphics chip is using your cpu ram, that's why you always see you have X amount of ram, but only a lesser amount of "useable" ram, the missing part is dedicated to the integrated graphics
now if its gaming with a dedicated graphics card, like one going into a pci-e slot, then ram speeds wont change a thing most of the time, BUT, if your cpu is being a limiting factor, faster ram will help it too, lets say you run a game at 40fps no matter what graphics settings you set, that means your cpu probably cant handle anything more than that, so if you're on single channel memory and install more ram, making it dual channel, or simply swapping slower 1600mhz ram to 2400mhz ram, your cpu will get faster, just not as much as an integrated gpu would
by the way, there's also "timings" for ram speeds, and they tend to balance out the overall ram speed, sadly, lets say you have 1600mhz ram and 1866mhz ram, the 1866 ram will probably have worse timings, meaning your actual bandwidth will be very close if not the same as the 1600mhz one, so no real upgrade there