Like i_krazyie_i said, RAM is where your data is stored once accessed by the HD.
The more RAM you have, the less time the computer will pass accessing the HDD so you will definitely notice faster load time between levels when playing games.
In theory, it is possible to mix one stick of 3200 with one stick of 2700 as your computer will treat it as another 2700.
The best though is to use brackets of the same speed and size, thus removing any possible conflict.
If possible, get the same brand name for your sticks as well, this ensures the best compatibility since they will also have the same specs (latency time etc...)
You might not notice a huge framerate increase by increasing your RAM size but you will certainly notice it for your load time and such and also when the video card requires more processing power (i.e. multiple enemies etc...) which cause a huge frame rate drop in games because your computer has to load new data in the RAM in order to process the data correctly.
The more RAM the better.
My gaming machine:
P4 2.8 GHz
2 X 512 MHz PC3200 in dual DDR mode
Intel D865PERL MOBO
integrated 6.1 MAX audio
Logitech duo MX mouse and KB
ATI RADEON 9800 PRO