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 core xeon 80486, 2.66Ghz.
Bus speed 1.33 ghz, SMC 1.7f10
Riser card s/n 922-7695
FB DIMMS 1GB 661-3930, FB DIMMS 2GB 661-3931

manual says i need RAM @ 667mhz, fbdimm, 240pin ECC,
36 devices max per dimm. not sure if needs FB or unbuffered.

 

thanks for the reply. partly solved some queries of mine.
can you send me a PM message, not sure how to send you one from here.