Upgrading my RAM: What should I buy?

Hey there, good people!

I’m currently running on 8GB of RAM, but I’m looking to bump that up by another 8GB. Right now, I have two 4GB sticks installed. I was wondering if anyone could offer some advice on what makes the most sense financially—should I go for a single 16GB stick or just pick up two more 8GB ones? Also, does anyone know exactly what type of RAM I need to ensure everything stays compatible with my current setup?

Here are my system specs:

CPU: Intel Core i5-3450 @ 3.10GHz (4 CPUs)
Motherboard: MSI-7758

Current RAM: DDR3 SDRAM, Memory Speed 800 MHz (DDR3-1600 / PC3-12800), Manufacturer: Kingmax Semiconductor

Thanks in advance!

Why doesn't that motherboard have 4 RAM slots?
Is that the B75 G43 board? I used to own one just like it.
If that's the case, just grab another 2x4GB. No need to swap out what you've already got.