Well it improves AE for ram previews and rendering, and if you wanna use multiprocessing.
As a 32bit app, AE can only get 4gb of ram to use per process. So with 4gb of ram on your system, minus what is needed by all the other background process (firewall, antiviruses, msn, blablabla) and Windows needs, you'll probably get around 3-3.5gb of ram for your AE process, and you won't really be able to do multiprocessing, because each core you wanna use, ie, each process AE will summon, needs at least 2gb of ram.
If you have 8gb, first of all, AE will be able to get it's 4gb max memory on a single process. But, not you can use more of this ram for multiprocessing rendering.
I'll give you as an exemple the configuration on my quad core Win64 8b ram PC:
I allowed AE to use 7gb of ram on my 8, allowing 1gb for win and other processes.
I allowed AE to use 3 cores, so each have 2.33gb of ram, above the 2gb limit, and i could of course have allowed only 2 cores with 3.5gb each, and that's what I do when i have very large assets, but most of time it's okay with my needs as I need more raw power than ram for large assets.
Hope that will help you make your choice
It will increase the duration of RAM previews, if AE is able to use all 4GB of RAM on its main instance. It may increase the speed at which the RAM previews are generated, because you can use multiprocessing or, if you already do, you may use more cores and those cores have more room to breathe. The same will be true for final rendering using MP. And of course generally AE will simply be more stable with more RAM. Again, if it can use full 4GB on its main instance to begin with, several problems people usualyl experience in low memory situations will "fix" themselves. And of course at some point you will also enjoy the ability to use e.g. Photoshop, while AE is rendering in the background (with reduced MP or MP off), something which is also difficult to do on systems with not enough RAM.