Hardware tests were made about what happens if you add RAM.
From 128Mb to 256Mb --> approx 50% more speed
From 256Mb to 512Mb --> 5-6%
512Mb to 1024Mb --> 2%
Depends of the setup and what you´re using it for, but the test was very diverse.
I upgraded from 256Mb to 512Mb and didn´t really notice improvement.
