WereCatf: No, adding more RAM won't do that unless your game actively keeps streaming stuff from the drive. Adding more RAM means the game can load more content in it so it has to load stuff less often, like e.g. in an open-world game where you roam around you'd experience fewer loading-times. However, if everything the game needs at the moment is already in memory then adding more RAM won't increase your FPS even by 1.
Cyraxpt: Uhm... Interesting, could that mean that it will reduce TW Shogun 2 loadings screen time? Because those 5m for each battle is what made me stop playing it...
Not really. You could put the whole game in a DIMMDrive ($30 program) if you have enough space. Then you'll see it load a lot faster.
You can see some improvements with things like stutter and maybe even some FPS in some games. Your computer likes to use your memory to pass things to your GPU and its memory, so having a big chunk helps that pipeline. That said, you won't notice an improvement for almost anything.
Marvel Heroes 2015 was a game that helped prompt me to use a 64-bit OS. Just that extra .75GB of RAM made a noticable impact on FPS in that game.
So, in general, adding RAM is going to do nothing. But if you're maxing out, which you may be doing with some software today, then adding RAM will certain add a benefit.
I'd go for the full 8GB, divert 2-3 GB into a DIMMDrive for loading my favorite game, and then enjoy it. That will give you faster load times (by a lot) and give you a little extra oomph for those games that like to cross that 2GB threshold memory (2GB Win7 Kernel + 2GB RAM).