RAM compatibility

I'm preparing to upgrade my PC using the new Sandy Bridge core processor coming out next month. I'm thinking of buying a motherboard made by Gigabyte (P67A-UD7) which will also be coming out next month. I'll be running it using my current PC components (an 850 watts PSU & 2 dual GPU Sapphire Radeon HD 4870X2 on quad CrossfireX set up). I been looking for some RAM modules to buy. The particular motherboard I'm going to use supports DR3 2133/1866/1600/1333/1066 MHz memory modules up to 16 GB of memory. I'm planning to put 4 GB of memory modules on each of the 4 memory slots. My questions are:

1) Hence I can't find a pair of 4 GB 2133 MHz memory module, can I use a pair of 4 GB 2000 MHz memory module or do I have to look for an 1866 MHz memory module?

2) In the motherboard specification, it supports a memory module rated at 1.5 volts. All the memory modules I've found were rated at 1.65 volts. Can I used a memory module with slightly higher voltage rating?

Are you planning on doing some crazy overclocks.? Otherwise, going that high on the memory frequencies does not make any sense.. You can actually be good with 1600 MHz ram modules..

Let the board launch.. They'll have a memory support list which will include all the memory modules tested and found compatible with that board.. You can easily choose any one from the list then..

Generally, as per the board specifications, the highest memory speed mentioned as supported without the acronym (O.C.) can be safely considered as the highest memory speed to opt for.. Memory modules running at lower speeds than that will also work fine..

you do realize that it's harder to overclock a full 4 slot RAM set.
unless you running flight simulator or CAD design then why do you need that much RAM anyways.?
it's better to run only 2 of the 4 RAM slots for optimal overclocking.
also some motherboards do not like a full RAM set but by the changes in voltage from clocking then it might stable out and post.

what emperus stated is good enough..