@sailor - you put information on your article that iMac has 4,5GB RAM ? is that true ?
yes, it is true.here
is specified only 2.5GB (0.5GB on motherboard + 2GB in slot).
Probably there were no 4GB memory sticks in 2005.
But if you use compatible PC2-4200 DDR2 SDRAM 4GB memory, it works (tested on OSX and linux).
I will send you exact type which I use.
Can you show the screen with OSX SYSTEM PROFILER and ACTIVE PROFILER with 4GB memory information?
For me, Morphos shows over 1.8GB, but OSX somehow does not see it. Neither 10.4 nor 10.5.
Maybe 10.6 will see?
It has been possible recently.