it can improve, but the memory also determines how fast/smooth your computer runs.
Premiere uses a lot of your computer's memory (AE uses even more), if you have other programs open/have a very complicated section in your video it can drastically reduce performance.
lets say that you are running these programs with your XP 2800 and 512MB of ram. Windows XP takes a chunk of your memory to run (probably around 200-300MB of ram). Now add on top of that premiere (in my experience, premiere takes about 100 - 150MB of ram), other programs take more ram...and soon, you're out of memory to use.
sometime before this happens, windows starts to use virtual memory (a part of your hard drive that is used like ram). virtual memory is a lot slower than physical memory (ram) which makes your performance even worse (but it beats the alternative)
if you try the same things with a better cpu (which takes care of tasks faster) and more memory (faster timings help too, but this isn't as crucial) you will without a doubt increase system performance.
I was doing fine with an Athlon 64 3200 and 512MB ram...but I upgraded to 1GB ram (keep in mind that this is a laptop I'm talking about) because I was getting memory access violations. but with AE, I wouldn't want to run it with anything less than 1GB of ram.
out of curiosity, what are your system specs?