It's ERROR CORRECTING CODE, it uses extra bits to ensure validity of data.
OK, any mixing (speeds or capacity) is not recommended. In fact alot of memory now comes in pairs since we now have DUAL CHANNEL.
You'll first need to find out what your MOBO uses, some mobos can use ECC and NON-ECC but, I seriously doubt you can use them together.
Next you'll need to know what speeds your MOBO can use.
Then shop around to see what you want to upgrade to. Like DDR RAMBUS comes in different speeds and the faster the mors it will cost. Not to mention that RAMBUS is already expensive which is why it is not very widely used.
That help??
