I have just replaced the two 1GB Ram chips in my iMac with two 2GB. That went well. What to do with the two 1GBs? Ah, my wife iMac. It had two 512chips. I replaced the 512s with the IGBs but on lighting up the machine showed one slot empty. I tried several times times but always with the same result. Yes, I checked the RAM types were correct. Yes the seem to be properly seated and the grey plastic removal arms are back in place. Any thoughts anyone?