The Z68 has four memory slots using dual channel memory. That means there are two channels or banks. If you opt for 2 x 8GB, fill one channel and you have the option to fill the other channel at a later date. Just do not put one stick in Channel A and the other stick in Channel B. Both in A and expansion for the future in B.
Why I generally completly embrace doing things like filling 2 RAM slots with future expansion capabilities, I'm not sure if that is best for a new Z68 system right now.
Right now 8GB stick are demanding a premium, and the fastest 32GB kits that G. Skill "certifies" for use with Z68 (I used Asus P8Z68 Deluxe RAM configurator on their web site as a reference) are 1600 MHz.
So, bottom line sugestion is to simply buy 4x4GB 1600 sticks now for your Z68 -- pretty affordable, and 4x8GB stick prices will surely come down if you want to upgrade later. Besides, 16GB really works pretty well and could be considered pretty balanced for your MB/CPU choice anyway.
Regarding OC'ing to 4GHz, 1600 MHz RAM with 1.5v should work just fine. Premiere Pro begs for lots of RAM - and yes, in the general scheme of people building PCs today, 16GB is already "lots of RAM" - but the actual speed and timing of the RAM does not really make any noticable difference in performance.
fill one channel
Careful here. If you fill up A1 and A2, you defeat the purpose of dual channel memory. You want to make sure you use up one slot for each channel first (A1 and B1), then you can add to the second slot for each channel later.
Thanks to all respondents. I think I'll go 4 X 4GB for now.
However, it would still be nice to understand which of Harm's "...fill one channel..." at a time; or Jim's "...one slot for each channel.." is the better approach if I had decided to go with just 2 X 8GB sticks??
This may turn out to be all about semantics, but with a triple channel motherboard (X58) you have 6 DIMM slots. With ASUS, they are designated as channel A, B and C and at the same time they are designated as bank 1 and 2. Confusing, right?
The DIMM slots are called A1, A2, B1, B2, C1 and C2 using the letters for the channels and the numbers for the banks. With only three DIMM sticks, you only use A1, B1 and C1, filling up the first bank completely and then later add another three DIMM sticks in A2, B2 and C2.
It is quite logical if ASUS has kept that coding on the four slots on 1155 mobo's, calling them A1, A2, B1 and B2. In that case you first fill A1 and B1 and then later add A2 and B2.
I hope this clears up the difference between channels and banks. Banks need to be filled completely to use memory best.
Thanks Harm, and all the other posters. That makes sense.