First of all! More RAM does NOT make your PC FASTER!
It man handle stuff better, thus giving an illution of working faster.. Faster RAM-modules may do better..
Then there is a balance between how much MEMORY your pc actually need for the stuff you use it for.. Adding more than that will in the end do very little! so "more ram" is NOT an universal solution!
But as said: It´s often the motherboards chipsets that set a limit of how much RAM the motherboard will handle! So begin with checking your MB´s limits.. And consider changing to FASTER RAM, if possible!