Hi Dawar Azhar,
Here at Dell we have tested this computer model with maximum of 8 GB of memory and obtained optimum performance. Benchmark of 32 GB for “Ivy Bridge” is set by the manufacturer of the processor “Intel”. If this works you can continue using it however, if the results are negative we recommend switching back to 8 GB only.
You can choose to buy RAMs of either type. We have tested both 1066 MHz and 1333 MHz. “Inspiron 5520/15R” supports:
DDR3 DIMM at 1333 MHz with Sandy Bridge Processors
DDR3 DIMM at 1600 MHz with Ivy Bridge Processors
Hope this helps.