Q: adding memory

You wanted to say that you want to add 8 GB of RAM more. You can buy a 8 GB of RAM module or 2 modules of 4 GB each one. If you think you will have to install more memory in the future, buy one 8 GB of RAM module. If not, buy 2 modules of 4 GB. Also, if you install 2 modules of 4 GB of RAM, you will get advantages with Dual Channel, improving performance.

You can buy the memory in OWC or Crucial > http://eshop.macsales.com/shop/memory/iMac/2010/DDR3_21.5_27 If you need help installing RAM, see > http://support.apple.com/kb/HT1423?viewlocale=en_US#1

The question you have to answer is will you be adding more memory down the road?

If you think there is a chance then definitely go with the single 8gb stick. The money you will save by not having to ditch memory if you upgrade will more then outstrip any performance improvement you might see (and I emphasize might, becasue unless you run benchmarks you're not likely to see a difference).

regards

You are welcome