what if you win the lottery and use the money to all the sticks on ram to fill the slots in your pc.
Well, assuming you have a motherboard that supports 16 GB of RAM and you filled it up...that would do...absolutely stuff for you.
There are 2 main concerns with RAM:
1. Speed
2. Do I have enough?
Purchasing faster RAM with lower timings (or modifying the speed in the BIOS) will provide a great performance yield than simple having a truck load of RAM.
As long as you have enough RAM, there is no compelling reason to add more. With Vista 2GB is typically enough. I will be going up to 4 soon however due to the requirements of some games.
If you really want to go crazy with your money, spend it on a better GPU. Thats where you will see better performance.