RAM upgrade: Quick question

I have a decent computer setup, but I’m looking to upgrade my RAM to 8GB and switch over to a 64-bit OS, which leaves me with a few questions regarding the memory.
*Just to be clear, I am by no means a hardware expert.

A moment ago, I popped the case open just to get a better look at what's actually written on the sticks.

2G DDR3 1333 DIMM CL9

I went off to see which 8GB kit would actually work for me, and I found this.
I've been browsing the web to figure out the installation process and how it all works, and I've run into all sorts of conflicting advice about matching memory speeds, processor compatibility, and various other technical specs.

So, if there are any members here who actually know their way around a motherboard, I could really use some guidance and advice.

Beyond just seeing "2GB DDR3 1333 DIMM CL9" on the stick, the manufacturer is actually listed right there too. Could you let us know which motherboard we're dealing with—specifically the brand and the exact model?

Is this really a priority right now?

It would probably be helpful if you shared the rest of your specs, yeah. Maybe you could just grab a single used 2GB DDR3 1333MHz module and pair it up to hit 4GB if you aren't doing anything too heavy. Otherwise, I guess you could go with those 2x4GB sticks plus an extra used 2GB stick to get yourself up to 12GB total...

NVIDIA GeForce GTX Series 2 x2GB RAM Intel
NVIDIA GeForce GTX Series 2 x2GB RAM Intel
Intel(R) Core(TM) i3-2130 CPU @ 3.40GHz

I'm not sure what else I need to get done.

How many memory slots do you have left? Or are you stuck with just 2?

Two slots available.

Yeah, I guess you'll have to ditch these and grab some new ones. Honestly, it doesn't matter much which ones you pick—just go for whatever's cheapest with the longest warranty. If you think you might jump to 16GB eventually, maybe just grab a single 8GB stick for now.