GDDR5 is "quad pumped" or for every clock cycle 4 bits are transferred instead of 1. DDR is double data rate so it's "double pumped". GPU-Z like CPU-Z reads actual clock speeds and not effective transfer rates. So your card is performing just fine since 900 x 4 = 3600 effective. CPU-Z reads your RAM the same way so DDR3 1600 reads as 800.
So short answer you are fine
Shader clock on Nvidia 4xx cards is always double the core clock.