Problems with Installing 4GB RAM on mid 2007 iMac

I ordered 2 X 2GB sticks of PC2-5300 DDR2 SODIMM RAM for my Boss's mid 2007 iMac from Crucial.com. Well they arrived a few days later and today I have been trying to install them for a few hours now without any luck. I have installed new RAM about 5-6 times without any problems before on different MacBooks and iMacs. After this install I plugged back into the wall and hit the power button, the iMac made a little noise (internal noise) then nothing, just a blank screen. I thought maybe the RAM was bad so I inserted 2 other chips (we ordered 4 X 2GB to upgrade 2 iMacs) and still no luck. After making sure the iMac would boot with the old RAM (which it did), I tried installing 1 X 1GB with 1 X 2GB and it booted up perfectly. I did this with all four 2 GB RAM chips to make sure they all worked and they did. Each time I successfully booted, I went to the system profiler to make sure it recognized 3GB of RAM and it did. So my question is why is this mid 2007 Aluminum iMac not working with 4GB of RAM, but it works perfectly with 3GB? Thanks in advance, sorry for the long question.