24 inch iMac mid 2007 with 2 x 2GB 667MHz SDRAM - can I install 2 x 4B chips?

I would go here and see what they have tested as working in your computer

http://www.crucial.com/store/listmodel.aspx?pl=iMac&mfgr=Apple&cat=RAM

The maximum your computer will accomodate is 6GB, at best your proposal will not work at worst it will cause Kernal Panics and possibly damage the OS. Navigate to www.macsales.com (OWC) where the sell a 6GB kit for your system.