FPS is affected by many things. IMHO network is one of those things that is never thought of but it does affect it as well. Some most notable are:
Processor, Video card, sound card, memory,some software, which runs in the background, video card settings, system settings, operating system itself, and lastly, game settings will all FPS.
Simply adding RAM only tackles a small part of the overall symptoms that affect FPS. Am I saying it won't? Certainly not. But as someone put out, how much IS enough, 4g is plenty in most cases. Video Ram which usually can't be adjusted is based on your video card.