Only you can decide if the extra memory will be worth the cost. AE will run better with more memory and 16 is better than 8, but it's processor power more than anything else that makes AE run faster. The performance increase as you add ram is not linear, it's a curve that rises quickly then flattens out. Sixteen GB is going to be better than eight by almost a factor of 2 but twenty four is going to be better than eight by less than a factor of 3. I hope you followed that.
i understand the curve
i dont have enough info to decide as i don t know after effect yet
all i know is that every time i tried to play with it it stucked(with 8 gb)
dont have enough knoledge to understand what will change with different setup.is important to know that cpu power is most important though
AE is going to be much slower than a NLE like Premiere Pro. There is very little that will be real time. More memory will give you a little smoother UI and a little longer previews. You will still have to learn how to adapt AE's performance to your project needs so that you may work efficiently and get things done. It's all about compromise. Even with the latest builds and a very powerful machine I seldom if ever do full resolution playback of long sequences and I seldom have more than one shot in a composition. I preview my motion and animation at lower rez without effects applied, I check critical frames when I'm compositing, and then I send the project off to render and work on something else because I have been at this long enough that I know what something is going to look like without having to do a real time full rise preview of the shot or sequence before I render.