That iMac takes max 16GB 1066 in 4 slots, so probably has 2x2GB and 2 slots free. Looks like the usual upgrades are to add 2x4GB 1066 or replace with 4x4GB 1066, so I would be quite surprised if 2x8GB 1333 will work.
It's late 2009 so don't expect any more High Sierra updates :( I also read something about a fan noise issue if you are replacing the original drive.