my iMac is aluminum and it came with osx leopard 10.5.4
Put your serial number into this website to find out when it was built:
http://www.chipmunk.nl/klantenservice/applemodel.html
Your iMac was certainly built in 2008; check out this version history chart for Leopard - 10.5.4 was released June 30th:
http://en.wikipedia.org/wiki/Mac_OS_X_v10.5#Version_history