Well 32-bit is limited to 4GB (3.0GB~3.5GB usable), next 8GB, 64-bit required, is perfectly fine for Gaming, next NEVER get individual stick(s) for pseudo sets. DDR3-1333 'can' be purchased individually, but it's nothing I'd do for a Gaming rig.
Note Windows Memory Limits ->
http://msdn.microsoft.com/en-us/library/windows/desktop/aa366778(v=vs.85).aspx
Sure RDIMM, Server ECC, can be added that way -- I have a box of ALL the same RAM in that rare example.
The Frequency depends on the platform, on SB and most any I can think of DDR3-1600 provides faster FPS over DDR3-1333. If you have an AMD then you'll need a 4-core to use much less gain from faster RAM.
Q - What EXACT RAM? ; provide links
Q - What CPU?
Q - What MOBO?