You have 2 or 3 RAM channels. It’s usually best to mount similar modules in the same slot of each channel (so say you have 2 8gb RAM sticks, you put one in slot A of channel 1 and the other in slot A of channel 2).
Below is a thread I posted last month after failing to upgrade with 2 more identical sticks. It’s tough. All the speeds need to be exactly the same–mine were and it still didn’t work. If they are not then you can manually dial back any sticks that might be faster than others so that all values are identical. I have read that some BIOS’s are more sensitive than others to differences. Apparently that was the case with me. If you run into snags, try booting with the new sticks only to rule out problems with the new memory. Be careful! Some stores will not accept returns unless the memory is defective. Amazon took mine back. Ultimately I bought 2x16s and removed my old 2x8s. Don’t worry too much about removing and installing. Your mobo will provide guidance on what should go where but I don’t think in matters except maybe for cooling.
Well, as long as it’s not a quad channel mainboard, you don’t get a bandwidth increase from using 4 sticks over 2. The layout of modules on the sticks themselves is what makes the difference. Anyways, that’s differences that you’d be hard pressed to notice in day to day use.