More ram or no if yes which one

I use my PC for gaming purpose, Its config is
ASUS LGA 1366 - Intel X58 - ATX Motherboard P6X58D-E
Intel Core i7 950
Corsair 6 GB PC3-12800 1600MHz 240-Pin DDR3 Core i7 Triple Channel Memory Kit
XFX HD 6870 x 2 (Crossfire)
3x1tb HDD
Windows 7 pro 64bit Genuine as OS

My question is
1. if I upgrade my Ram to 12 GB will it improve gaming performance. ?
2. If Yes, then the existing 2gb x 3 is discontinued. So can I Install instead 4gb * 3 along with my old ram and have total of 18Gb ram

Thanking you