Q: RAM Questions

You need:

Do not use ECC RAM. You can purchase the required type of memory at OWC or DataMem, both of whom have memory specifically for Mac models like yours.

You can't use ECC memory on an iMac because it's not compatible. You can find all the requirements in this support page > http://support.apple.com/kb/HT1423?viewlocale=en_US

Furthermore, I only recommend to get memory from OWC or Crucial, because other memories can cause problems with the iMac, although others may work without any problem. Obviously, you have to buy 1333 MHz DDR3 RAM for your Mac. Depending on the iMac you are using, you have different memory sizes

Apple memory is expensive, and that's why I recommended OWC or Crucial. Both have got memory compatible with Macs that won't give you any problem, and you will save money. And of course, that memory will work with your computer

Yes. If you install those you will increase your RAM from 8 GBs to 16 GBs.

You are correct

Correct

Yes, those will work if you wish to upgrade to 32 GBs. That package consists of 4 modules which replace whatever you now have installed (2 modules.)

You are welcome