You cannot simply say adding RAM will help the computer speed, a computer wont be increased by adding only one thing, unless that one thing is the only thing dragging it behind, all components need to be to-par with each other.
Windows Vista has a nice tool for finding what would be good to update, it has something called "Windows Index" and gives your hardware a rating between 0 and 6, with 6 being the highest and 0 being the lowest, and then you simply update the lowest ones
