it depends on the psu you have and the models you are seeing, 3, 4, 6 or 8 gbs models both from amd and nvidia
if you are a nvidia fan, the 6 gbs is what you should get
if you are a amd fan, 4 or 8 gbs is a good option
about the bottleneck, the cpu is not strong but should let you have a decent experience but consider in the future to get a i5 at least