Upgrading My RAM - Compatibility Questions

Hello all,
 
I've done business with AVA Direct about a year and a half ago (oh how time flies!) and am still satisfied with my build, but I'm looking to upgrade my RAM. I currently run 4GB, and am looking into upgrading to 8 or 16GB. (Or 12 if I really have to.) I've been running into issues that include programs (Firefox, for one in particular) consuming the majority of my available memory until the application crashes. My solution was to upgrade the RAM, as this would probably assist with multitasking a bit as well (correct?) I apologize if I'm in the wrong category for the forum, I just currently have some questions before I settle on what I decide to buy.
 
My motherboard is currently discontinued, and that raises one of my questions. It was an Intel DH61CR, specifically this according to my order:
DH61CRB3, LGA1155, Intel® H61, DDR3-1333 16GB /2, PCIe x16, SATA 3Gb/s /4, VGA+DVI, HDA, GbLAN, mATX, Retail
 
My currently installed RAM are:
CRUCIAL, 4GB (2 x 2GB) PC3-10600 DDR3 1333MHz CL9 1.5V SDRAM DIMM, Non-ECC
 
So, my questions are:

• For the RAM that I upgrade to, does it specifically have to be some sort of DDR3-1300MHz SDRAM DIMM Non-ECC, or can it be some other variant? I'm looking to keep the voltage the same @ 1.5V, and my motherboard has two RAM module sockets if that helps at all.

• Are ALL new modules compatible with my motherboard (in the DDR3-1300 yada-yada-yada category)?

• Would mixing brands or perhaps replacing my Crucials with a different brand cause any issues?

I thank you for your time and assistance!
 
-Mark