Forgive me, i'm a newb at this and this is my first PC I will be building so I want to make sure I get it absolutely right, with as little or no rework if possible.
Looking at purchasing memory kits for:
Asus Rampage III Extreme motherboard w/ Intel 990x Extreme i7 CPU
On the manufacturer page it says this:
6 x DIMM, Max. 24GB, DDR3 2200(O.C.)/2133(O.C.)/2000(O.C.)/1800(O.C.)/1600/1333/1066 Hz Non-ECC, Un-buffered Memory
Triple Channel Memory Architecture
Supports Intel® Extreme Memory Profile (XMP)
* Hyper DIMM support is subject to the physical characteristics of individual CPUs.
The CMX12GX3M3A2000C9 is crap - don't use them. I use the CMT12GX3M3A2000C9 and they're great.
It depends on what the 'use' of your PC; for 'Gaming' sure the DDR3-2000 adds a few FPS but they're not for the newbie because they require you to OC at minimum the BCLK to 160MHz or higher.
The CMP24GX3M6A1600C9 are fine, the CMZ24GX3M6A1600C9 is cheaper and performs the same, and you might want to see this video -> http://www.youtube.com/watch?v=5AOylHaCyUA or remember Memory Configuration Protect -> Enable.