Contrary to what a lot of people think, RAM does not improve performance in the an obvious sense. RAM is simply faster memory that your processor (any one) goes to when it needs something. This can improve performance and response, but there's a stopping point to where this doesn't work.
For instance, if you are running programs on a system with 128MB of RAM, and the programs take up all 128MB with say 128MB in the swap file (a section on the HDD that, to the rest of the system, looks like RAM), performance is slow and response time is high because programs need to access the swap file, on much slower memory, quite often. In other words, programs are stuck because they need to wait for data.
If you add 64MB to the system, then the system response and performance improves as it has to look at this swap file less.
Add another 64MB, and again, it improves, but it may not be as much (Say 64MB of stuff in the swap file wasn't as needed as often). Adding any more RAM will do nothing, because everything the programs need is already in RAM.