Personally, I think it's a matter of rounding laziness on the part of the marketing hacks who write up all the press releases. Now the RAM vendors just remark the same 10666.67 parts with three different labels so customers can buy the one they think their board requires.
Considering how clock signals are generated, I can't imagine how you'd easily get 10600 MB/s.
Just to clarify what these numbers mean, a 666.67MHz memory clock is double pumped (as in DoubleDataRate DDR memory) to get 1333.33 million transactions per second. All current x86 CPUs have a 64 bit memory bus so 8 bytes are transferred with each transaction. 8Bytes x 1333.33 million transactions = 10666.67 MB/s
If you are just looking for a good brand then Corsair or GSkill are hard to beat.
You can get 4GB for about $30 and 8 GB for $40 and your board will support 8GB of 1.5v DDR3 1333.
If you are running a 32bit version of Windows then you will be limited to 4GB of RAM. That is all any 32bit OS can use. If you are running a 64bit OS then go for 8GB as it's so cheap.
If you are not sure if it's a 32 or 64bit OS then if you are running Windows XP there is a 99% chance it's 32 bit and to check Windows 7 go to Start > Control Panel > System and it will tell you there.
Yep, I had 2 Core2 chips I overclocked. An E6600 I got from 2.4Ghz to 3.4Ghz and an E8400 from 3.0Ghz to 4.050Ghz. They are very easy to overclock. You will need to start with aftermarket cooling. Do you have a good CPU cooler?