If you use RAM with different speeds, it will usually use the lowest RAM speed for all of them.
If you add a single 512 to your existing 2x256, you may need to put the 512 module in the first slot (closest to CPU) for best results.
Not sure about the pairing, on this system I've got 4 slots which must be paired up to work efficiently. But, you have 3 slots? which may mean you don't need to pair the modules up. It should tell you in the mobo user manual whether you need to pair the RAM modules.
If you add a single 512 to your existing 2x256, you may need to put the 512 module in the first slot (closest to CPU) for best results.
Not sure about the pairing, on this system I've got 4 slots which must be paired up to work efficiently. But, you have 3 slots? which may mean you don't need to pair the modules up. It should tell you in the mobo user manual whether you need to pair the RAM modules.