iMac model 7.1 memory issue

My girlfriend has a 2007 model iMAC, 2 GHZ Core 2 Duo with 1 GB memory installed. I used the memory scanner at crucial.com to scan and then order the memory indicated.The first module sent did not work, returned it and the second module sent did not work either. I installed it in the 2nd bank and it was not recognized, uninstalled the memory in bank 1, tried the new memory in it and the computer would not boot. The memory is a 1GB 200 pin DDR2 Sodimm 128Mx64 PC25300. The tech said this is the correct memory. Does anyone know what memory will work in this iMAC model and where to obtain it. Thank you

 

All she does with her iMAC is email and internet browsing and a few lightweight spreadsheets. I am thinking 2GB would work fine for her, on that thought can I put a 2GB module in bank 2 and leave the 1GB in bank 1 for a total or 3GB or do they have to match. Thanks for your help.