Currently Being ModeratedFeb 10, 2013 9:03 AM (in response to Dianne.m.Hewitt)
Welcome to the Apple Support Communities
The maximum memory that a Mid 2007 iMac supports are 6 GB of memory, so install those 6 GB of memory. You can buy the memory in OWC or Crucial (Crucial is cheaper and offers the same compatibility warranty as OWC).
If you need help installing the memory > http://support.apple.com/kb/HT1423?viewlocale=en_US#2
Currently Being ModeratedFeb 10, 2013 9:14 AM (in response to Dianne.m.Hewitt)
Ideally you should load ram equally but your Machine and that total only allows for a 4GB + 2GB. I have the same model as you. She runs well with 6 GB and it is quite simple to load.
Do check out you existing programs, apps and games before diving into Lion. It does not have the ability to work with Rosetta based programs.