Unless your system was memory-constrained to begin with, adding more memory won't help framerates. You can confirm this by pressing F3 in game and checking the memory usage in the upper right hand corner.
Installing a decent graphics card into your system will solve the issue.
That CPU will more likely bottleneck the GPU than the other way around, hence why I wouldn't recommend anything over an R7 250 or GTX 650. Your E3400 processor is a low end CPU on an old socket (775) so it wouldn't make sense to put any money into upgrading it.