Q: memory for a late 2009 imac

Hello,

Everything is explained here : http://support.apple.com/kb/HT1423#1

You can have up to 16 GB memory.

Use Small Outline Dual Inline Memory Modules (SO-DIMM) that meet all of these criteria:

Hope this will help.

One of the more popular sites among Users of these Forums for reasonably priced, stable RAM is OWC

      http://www.macsales.com 

I've successfully used them for years and their Customer Service is great.

Hope this helps