Confusion regarding RAM

Information:
1) The i7-2600k processor page suggests it supports 1333Mhz Memory
2) The motherboard page says it supports 2000Mhz+(OC)/1600Mhz(Normal) Memory
Eg(In my case): Asus P8Z77-V PRO OR GIGABYTE GA-Z77X-UD5H-WB WIFI

Questions:
1) DDR3 RAM Sticks with what MHz frequency should I buy?
2) At what Mhz frequency will they run under normal mode(i.e. w/o Overclocking)?
3) To what Mhz frequency will I be able overclock those DDR3 RAM Sticks with the aforementioned Processor and Motherboards?

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Information:
The ASUS p8z77-v pro page says :

Memory
4 x DIMM, Max. 32GB, DDR3 2600(O.C.)/2400(O.C.)/2200(O.C.)/2133(O.C.)/2000(O.C.)/1866(O.C.)/1800(O.C.)/1600/1333 MHz Non-ECC, Un-buffered Memory
Dual Channel Memory Architecture
Supports Intel® Extreme Memory Profile (XMP)

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and the GIGABYTE z77x-ud5h-wb wifi page says:

Memory
4 x 1.5V DDR3 DIMM sockets supporting up to 32 GB of system memory
* Due to Windows 32-bit operating system limitation, when more than 4 GB of physical memory is installed, the actual memory size displayed will be less than 4 GB.
Dual channel memory architecture
Support for DDR3 2400(OC)/1600/1333/1066 MHz memory modules
Support for non-ECC memory modules
Support for Extreme Memory Profile (XMP) memory modules

Click to expand...

Questions:
Notice any difference?
1)Why does it say that GIGABYTE only gives one OC frequency and ASUS so many?
2) Does that mean GIGABYTE can be OCed to any freq upto 2400Mhz or does it mean it can only be OCed to 2400MHz?

Click to expand...