OK, here's an update.
I couldn't get memtest to start up, so I just went with OCCT, which is a pretty cool program! So using OCCT, I tested each RAM (from here on, calling them RAM 1 and RAM 2) in each slot (bottom slot and top slot). RAM 1 performed marvelously in both slots, and so did RAM 2. I did OCCT on RAM 1 in both slots, and RAM 2 in both slots. Large data stream, for 15 minutes each. Both worked great, didn't freeze or anything. CPU temps hit 38 C, MB hit 34 C, so nothing out of the ordinary there. Along with OCCT, I did som browsing in Firefox, opened up so music files, looked around the computer, etc. No problems.
So, after that, I put both RAM sticks back in, and started up. It started fine, opened up all programs at start fine, and was working great. At 6:50 up time, it froze. Temps were in the normal range (31 C for both CPU and MB), there was never any spikes in memory or CPU (I opened task manager to check this), things were running just as they did with single sticks in, but it still freezes.
So, I'm taking this to mean both RAM sticks work, both slots on the MB work, but for some reason, these two things don't work well together. Also, the CPU works fine, as I was maxing it with OCCT on five separate tests (I did RAM 1 in bottom slot twice) and it got hot, but kept on going.
Calum, I'd like to try what you mentioned about checking memory timings and voltages. If you could elaborate more on this or post a link on how to do it, I'd appreciate that. I've never really messed with BIOS before, but now's as good a time as ever.