Adding RAM beyond 4GB isn't going to help FPS in games much at all, games just don't make much use of more than 3GB of RAM. Overclocking your CPU and/or getting a second/different video card are the main things that will increase your FPS. What games do you play and at what resolutions and settings?
Yeah, more RAM won't help you. If you want more FPS either (first option) overclock your CPU, or (second option) get a better graphics card. I don't recommend the second option because you have a decent graphics card as is, and your motherboard probably doesn't support SLI unless it has a Nvidia chipset, which means you would have to get rid of current card instead of adding a second one in SLI. What FPS are you getting now?
The question you should be asking is "Does my computer need more memory to run efficiently?".
How you answer that is by using performance analysis of the computer - eg: using performance monitor in Windows.
Examine the computers PEAK memory requirements.
If the computer is using more memory than it has physically installed - then the answer is "Yes" - increasing memory will improve your system's performance. IF the answer is "No", then you will be throwing your money down the drain and it won't achieve anything.
If you want detailed info on how to do windows performance monitoring, then do some searching on MS Technet and find the articles on examining memory performance and what the various counters actually mean, as a lot of folks misinterpret the meaning of various OS perfmon counters. It's not hard to do, but you should properly understand the performance metrics and what they mean.
You could also investigate if the game you are concerned about supports addressing memory higher than 2 GB and if you are using a 32 bit OS, you might want to consider changing the boot.ini settings to allow user level processes to address up to 3 GB of memory, rather than the 2 GB default per process limit.
There's some easy to find tools for altering game executables to make them aware of memory greater than 2 GB.