macpro 1.1 (2.1) 2x8 or 8x2GB PC2-5300x 667mhz Ram combinations

hello,

I want to buy 16GB RAMfor my 2006, 1.1 (2.1) MP DC device, with 8GB ram (in all risers)
so i can install ElCapitan.

I found some ads, selling either 2 DIMMSx8GB or 8 DIMMSx 2GB, PC2-5300x 667mhz RAM.
Which one is best and possibly work faster, or it does not matter at all...

What the term i.e. 2Rx4 means ?

What is difference from 5300G, 5300F ram. Can xxxxG and xxxxF RAM work ok on my mac?

Further machine info is:
Kernel version= darwin 11.4.2, with secure virtual memory
dual...

macpro 1.1 (2.1) 2x8 or 8x2GB PC2-5300x 667mhz Ram combinations