According to the specs listed, you currently have 4Gb of DDR3 RAM at 1333MHz. Typically you'll want RAM that matches the MHz speed. You're motherboard is set up to use 1066 or 1333 RAM. You're running windows 7 64-bit so you can max out at 16GB of RAM for this board.
If you have one RAM card, I would find another stick of RAM that is 4GB DDR3-1333.
If your PC currently has 2 RAM sticks, I would find 2 2GB sticks of DDR3-1333 RAM. You can find these sold together and is better as they are matched in timing as well.