ASUS Rampage III Extreme mobo & Memory question

Hi all,
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.

Looking at purchasing CORSAIR:
CMX12GX3M3A2000C9

DDR3-2000 (XMS3-9-9-9-24*1.65v) Triple Channel

12GB Kit (3 x 4GB)

1.65v

9-9-9-24-2T

OR

CMP24GX3M6A1600C9

DDR3-1600 (Dominator-9-9-9-24*1.65v) Triple Channel

24GB Kit (6 x 4GB)

1.65v

9-9-9-24-2T


Now my question is, should I buy two kits of the 12GB 2000Mhz or just go with the straight 24GB kit right off the bat?

Thanks for all your advice and help in advance!!