Well,
Right now, I had a extra 1gb. I can always upgrade to the newer OS and ram down the road. Is there a cost to upgrade to Mavericks ?
Good work & congratulations on your new/old iMac.
I looked up your model on EveryMacs.com, here are the specs. Your specific iMac unofficially supports 6GB RAM.
Yes, Mavericks is a free download through the Mac App Store, the icon should be on your Dock or in your Applications folder.