Pacopag :
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Patriot DDR3 Viper II Dual Channel - 1333 or 1600.
Why does the Viper II specify that it is Dual channel and the others don't?
The Viper II Dual Channel is sold exclusively in pairs, which are guaranteed to have identical specifications. The others may be available as single modules.
For big numerical computation tasks with large datasets that will hammer the memory quite hard, such as medical imaging or large particle simulations, I'd be inclined to use ECC memory. (And yes, dual-channel is a must for these kinds of tasks.) You will need an AMD processor or an Intel Xeon workstation/server-class processor to use it as the memory controllers in Intel's consumer chips (Core/Pentium/Celeron) don't support ECC as far as I am aware. (Some Xeons even have triple-channel memory controllers.)
For computation with a small dataset (CPU-bound iterative stuff like calculating pi to umpteen zillion decimal places) you'd be fine with non-ECC memory. There would probably be little benefit to using dual-channel memory if the dataset will fit into the CPU's cache, but memory is so cheap these days that you'd be silly not to have it anyway - you'd be "spoiling the ship for a ha'peth of tar" if you scrimped a few dollars/whatever by only fitting single-channel.
Hope this helps,
Stephen