Yes, this product is compatible. However, RAM modules should always be added or replaced IN Pairs, so it is better to buy the following item straight away:
Corsair CMSA8GX3M2A1066C7 (2x, 4GB, DDR3-RAM, unbuffered)
Also pay attention to the arrangement when installing (for optimal performance!): Always place the same module pairs (e.g. if two new modules are added to the already installed ones) "above, left and right" or "below, left and right"!
This iMac supports up to a maximum of 4x 8GB = 32GB.