To know if you'll see an improvement, you can run the following test:
While you're doing whatever, run task manager in the back round. If your RAM usage is 90-100 percent, then the RAM upgrade would help you. Otherwise, it's best to put that money towards a new CPU or maybe even GPU depending on what you're doing.
To know what you'll need to upgrade next, here's another experiment:
Get HWiNFo. It will monitor just about everything you'll ever want to know about your system except FPS.
Like task manager, run it in the back round while you doing your stuff.
Take note of the percent usage on:
Each CPU thread
The RAM
The GPU
Whichever one hits 90-100 percent is what you want to upgrade.