Reduce memory usage in OS X Lion
I doubt you have summer 2006 iMac. Those models are all Core Duo and don't support Lion. And by the way they don't support more than 2GB of RAM anyway.

Only thing you can do about memory usage is to turn any service in System Preferences that you don't need especially under Sharing tab and watch for autologin items in Accounts tab.

Also you may uninstall Adobe Flash to make browsing more responsive. Or go back to Snow Leopard or even Tiger(though it's poorly supported now).