thank’ s to your guides... and tools, i managed to change a corrupted hard disk to my wife’s iMac and i installed a 8 GB ram modules to my macbook pro. unfortunately the ram didn’t work as supposed and i reinstall the 2GB ram.
for the iMac i used as a replacement a 1TB WD RE4 drive. it was a bit difficult and took me about an hour but now the iMac is really very fast... faster than before i could say.
for my macbook pro the process was really easy..
for any iMac you want to fix or upgrade, i recommend not to disconnect the screen power cable... just find something or someone to help you hold the screen next to your imac without stretch the cable.