You may want to check your motherboard and CPU before you consider RAM.
Any modern motherboard supports at least Dual-Channel RAM. This is where, out of the 4 RAM slots, slots 1 and 3 are Channel 1, and slots 2 and 4 are Channel two. If you have two sticks of 4GB RAM, try putting one stick in a Channel 1 slot, and a second stick in a Channel 2 slot. This way, instead of both sticks sharing a single bus to your Memory Controller Chip (MCC), they both send data simultaneously instead of taking turns. If your motherboard does not support dual-channel, it won't boot.
Further information about your motherboard: It doesn't matter how fast your RAM is if your motherboard operates at a slower speed. Your motherboards speed will determine how fast all of the components operate at, and this includes your RAM. If your motherboard works at 1333MHz then your 1600MHz RAM will only work at 1333MHz.
Your CPU is important because the processing power determines how quickly or efficiently it will process the data handed to it from the RAM. Obviously a more powerful CPU will be more efficient for this.
Your GPU usually uses its own built-in RAM. The only situation in which your GPU will utilize the RAM is if you have it set to use Virtual RAM. This is normally set in either your system BIOS or your video card's software (normally installed with the CD that comes with it for the drivers).