Ram Support for the Motherboard.

The OC means that you will manually need to change the RAM settings in order to run at that speed. Generally the only reason to get faster RAM is if you are planning a serious OC on your CPU, since OCing the RAM does very little for performance.

First of all, you didn't specify the motherboard.

If we assume the X58 (Core i7) then the answer is 1066MHz but overclockable to 1333MHz.

If we assume an AM3 board with DDR3 then unregistered RAM might be 1333MHz. If your motherboard supports 1600MHz AND it's recognized then you'll get 1600MHz. If it's not recognized you'll need to set it manually, though BIOS settings sometimes add support for newer RAM so you can use auto mode again.

I notice they sell 2000MHz RAM for Core i7 and I can't find any mention of how to use it unless you are massively overclocking your system in an insane way. This does not occur to be the case, it seems more likely that the memory controller and CPU core speed can be independently adjusted. If so, I just can't see a Core i7 920 needing the 2000MHz memory. It's really confusing even to an Electronics Technician like myself. It's easy to understand if I stumble across a good article but I haven't yet.

2000MHz can be supported normally on certain AMD setups. The AMD Phenom II X4 955 supports up to 2000MHz (the memory controller is on the CPU). The motherboard must also support this otherwise it will be downclocked.

A Core i7 setup uses three sticks and most everything else is dual memory support. The overall memory bandwidth for Core i7 is impressive but it boils down to whether you need it or not. Memory bandwidth limitations only usually come into play when the CPU has all the cores maxed out such as when compressing or transcoding.

It's difficult to show a difference between 1066MHz and 1600MHz in most usage, and likely never for any current video game.

This motherboard has full support for this CPU. Some motherboards require a BIOS update to recognize new CPU's in which case they must already have an existing CPU to do so. After such a BIOS flash they would then insert the new CPU which would be automatically recognized, no setting changes required.

Again, according to ASUS full AM3 CPU support is there already.

RAM:
Any DDR3 set will work. However, you may not be able to attain 2000MHz. According to Asus this motherboard supports an overclock of up to 1600MHz. If the price isn't much difference 2000MHz RAM is fine to buy. If there's a significant difference, get the 1600MHz.

For most things, such as games it's difficult to be limited by your memory bandwidth. This only tends to occur in specific situations such as using all four of the CPU's cores at 100% and even then it may only occur under a CPU overclock. Basically, it's not an issue for most people and likely never for a gamer.

Summary:
Yes, everything's fine. I'd get 1600MHz DDR3 if the price is significantly cheaper. Read reviews, not all RAM's the same. Always err on the side of quality, not the highest speed (which you likely can't utilize anyway).

FYI, you should be getting 2x2GB of DDR3. Any more is a waste without a specific need like editing massive Photoshop files. You can find exensive benchmarks and discussions on this but basically 4GB is optimal. You can always add in another 2x2GB if things change in the future.