Correct memory speed for 580 1333 or 1067 MHz?

I am planning on adding memory to my six year old Inspiron 580 to get a few more years of life out of it.

When I enter my service tag and run the parts finder, Dell suggest adding DDR3 SDRAM - DIMM 240-pin, 1333 MHz ( PC3-10600).  However, when I ran the Dell system detect, it said my memory is DDR3-SDRAM (1067 MHz)?

Also, the Dell web site says my maximum configuration based on my service tag is 8 GB but the documentation with my 580 says 16 GB.  Since I am running Win 7 64 bit, I assume 16 is correct as it agrees with Microsoft on Win7 64 bit OS.

In a related question, can I add a 4 GB DIMM with the other three 2 GB DIMMs (10 GB total) or do I need to use a 4x2+2x2 (12 GB total) configuration.

Thanks,

Scott