Thank you for your input.
Aside from selecting from a reputable vendor and making sure the specs were correct, I was not putting much more thought into it.
16GB (4GBx4) total is my target and within my budget. My current RAM is the stock 3GB from three modules.
I did not even know non-ECC was an option in a "Workstation" class Macintosh. As such, I only see ECC RAM advertised for it. I know in the PC world, if you try to install non-ECC RAM in computer that expects it, it will give you warnings in BIOS or may not even POST at all. I can tell you I will not be performing any mathematically critical computations. The most intensive tasks I will be performing is video editing/playback and video encoding with HandBrake.
You are saying there is a performance hit if all four of my slots are used? Interesting. I did not expect to hear that. Is that fact even documented anywhere in Apple's literature?
ECC is not a must on cMP. In fact, the system won't say anything but work as usual, apart from ECC disabled.
Normal PC DDR 3 RAM works, which is a cheaper option. Anyway, I won't recommend anyone do this. I did it just because I am too lazy to get the correct RAM. And I have no mission critical stuff on this computer.
However, you better know that the X5660 only has 3 memory channels.
No matter how good the cMP is, it can't change the CPU's spec. In fact, as the others points out. It's been discussed many times already, the 3rd and 4th slots share the same channel.
If you use all 4 slots, by running benchmark, the memory performance can easily drop 30% or more (much much bigger hit than 1333 CL 9 drop to 1066 CL 7). However, in real world, for most of the applications, you can't feel it.
If you want to maximize the memory speed, you should not go for 4x4G, but may be pay a bit more to go for 3x8G. Or if you believe that 12G is good enough for you, 3x4G is also an option. I mainly use my machine for video editing, photoshop, media server, web surfing etc, with 2 profiles constantly logged in. The memory used usually at or above 24G.
If you want best performance, 24G should be a better option for you. Technically, your RAM will run much slower at 4x4G config, and your Mac can easily running out of RAM if only 12G available (that means, big performance hit).
If you are not willing to take that <3% performance hit from 1333 to 1066, I don't think that you are happy to take that benchmarked >30% performance penalty.
Anyway, I suggest that you study a bit more before you make the decision. It seems you are not quite clear what you can do, and what will actually happen.