My mobo Asus
M4A78LT-M LE says "The motherboard supports DDR3 memory that features data transfer rates of 1800 (O.C.)/1600(O.C.)/1333/1066 MH" What does the (O.C.) mean? Does it mean i have to overclock for it to be able to support that memory?
Is the M4A78LT-M LE the motherboard you have now or the one you're going to buy?
It means that the board supports 1066MHz and 1333MHz auto detect or by manually configuring it to the suppliers recommended parameters in the bios. However you can run 1600MHz and 1800MHz and maybe higher via a overclock. The Gskills and OCZ ram you was talking about earlier (maybe in another thread) will typically overclock to those speeds without issue.
Edit:
It's the board i'm going to buy. So the cpu is overclocked so that it can use the ram at 1600 MHz speeds?
If the memory is rated at 1600MHz by default you shouldn’t have stability issues overclocking the motherboard to the rams rated speed of 1600MHz
If you are trying to overclock say a 1333MHz module to 1600MHz or a 1600Mhz module to 1800MHz+ stability will depend on the motherboard, memory brand & model, compromising on latencies, heat, voltage etc.
Bear in mind as you overclock the processor via the FSB the memory's bus will also OC too synchronously. There will also dedicated OC'ing section specifically for the memory. There are plenty of articles on overclocking around here I would advise becoming familiar with it on a theoretical level before pissing around with the bios.
PS. The M4A78LT-M LE is a good board, but be careful, it's still a old 760G chipset and it only has 2 memory expansion slots, which might hinder you from upgrading to 8GBs of ram in a 2GBx4 configuration in the future. (unless you are prepared to throw your memory away for 2x4GB). Just a thought.