As I noted in the edit of my post, the matching pairs does not apply to your machine. RAM slots over the years have had a cyclical requirement. A long time ago, with SIMMs, you often would have to upgrade in sets of two or four. I don't recall if this was technically required or a performance issue. Later, as the technology improved, you could have mismatched sets. If I recall correctly, that was with the advent of DIMMs. Then speed improvements with chipsets and motherboards changed things back to favoring matched pairs of DIMMs. In your machine's case, since there are only two slots, and Apple shipped them with a single slot filled, it is not required to install in pairs. My old 2002 iMac was like that. In fact, it had two completely different types of slots. One was for notebook DIMMs and was user upgradeable, the other took traditionally desktop DIMMs and was only upgradeable for the adventurous.
Your machine officially only supported 4 GB of RAM (2x2GB) when it shipped, but as memory density improved, it was found that you could install a new 4GB DIMM in one of the slots to increase the max to 6 GB. Something about the motherboard chipset must prevent addressing of a full 8GB. My current MacBook (the last of the MacBook line) Officially supported 4GB, but later the increased density RAM came out and now it has 8GB in it. Who knows, the new 27" iMac may eventually be capable of 64GB if 16GB DIMMs are released and if the chipset can address that much.
Flash
The Flash video problem might not be related to your RAM at all. Flash is well known to be a generally poor quality language, though I'm sure it was cutting edge when first released. Apple doesn't even support Flash on any iOS device as a result. I play a few Flash based games, and nothing else I do comes near maxing out my CPU for as little as is actually going on as Flash games do. Encoding video (albeit not super fast) doesn't even cripple my computer like some Flash games.