If this is not the memory supplier as installed, it would be prudent to check out the links below. Buying memory because it is cheaper or your friend uses it or whatever is not good practice with Apple products.
Many people on these forums speak well of the following memory suppliers.
In North America contact OWC … http:// Upgrade Apple iMac Memory
In Europe contact Crucial.com … RAM Memory Upgrade
Both companies have facilities to guide you and will recommend the correct memory package to meet the needs you describe to them. There may be benefits buying from whichever company has a branch in your own geographical location insofar as Shipping charges, Local sales taxes and Exchange rates. Check out the basic memory costs and do your sums.