When adding new memory, you should match what is already in your system. Adding non-ECC memory to an ECC system will disable the error-checking and correcting ability of your memory modules. While your system may still operate, the enhanced features of the ECC modules will no longer be active in your computer.
You can determine if your system has ECC by simply counting the number of black memory chips on each module. ECC (and parity) memory modules have a chip count divisible by three or five. This extra chip detects if the data was correctly read or written by the memory module. If the data wasn't properly written, the extra chip will correct it in many cases (depending on the type of error). Non-ECC (also called non-parity) modules do not have this error-detecting feature. Any chip count not divisible by three or five indicates a non-parity memory module.
Using ECC decreases your computer's performance by about 2 percent. Current technology DRAM is very stable, and memory errors are rare, so unless you have a need for ECC, you are better served with non-parity (non-ECC) memory.