Adding a LOT of RAM does not really increase the game's FPS.
If you want to increase game's FPS,you need a good graphic card with good amount of memory and good GPU which can process all that graphic memory fast enough.
You can add as much as RAM you want,but it will make no difference.
For example if a game runs under 15 FPS on 1 GB of RAM and then you upgrade RAM to 4 GB,game will still run under 15 FPS.
RAM does not make games run faster.It only runs them more stable because it is smaller chance that the system runs out of RAM resources.
You can try MemTurbo and see how game runs with it.Because it has CPU ROCKET and that can help some games to run faster.
Still if you want great speeds,you NEED a good graphic card and NOT 8 GB of RAM lol.
I ran Resident Evil 4 and 5 on one of my test computers under just 512 MB of RAM and the speeds were great,but that was because I had a good graphic card with good GPU and good processor.You said you have 6 GB of RAM..........Well believe me...you do NOT need more than that unless if you are playing 20 games at once lol.
Cheers!