Will 1600 Memory work in Late 2009 iMac

The iMac will down clock RAM to it's MAX speed but I am not sure if there is some sort of limit to how far "down" it will do so.

I placed some 1866MHz RAM in a machine that only goes to 1600 and it down clocked the RAM to 1600 automatically.

Here are 2 links to Amazon with 1066 RAM.
4GB Kit
8 GB Kit

 

Always put the ram modules in pairs. You can also put the 1333 ram modules in there. I believe the latency is also better than 1600 ram. They are pretty cheap now a days, about 20 Euro for a 2 GB module. So start with 2 x 2GB would be my advise, depending what you're going to use it for.