Only one suggestion.
Take it to an Apple Genius Bar as you have a DEAD Mac and only Apple can fix it. I'd leave out the part where you installed extra RAM in the system and just say "I started it up and it DIED".
If you didn't buy the RAM from Crucial.com then when you get your Mac fixed or a new one order from them as they guaranty the RAM they sell will work.
Looks like its moy only option. Still, I thought the RAM was supposed to be user upgradable on the 27 iMac - that is one of the reasons I choose it. Just don't understand how a simple RAM upgrade could render the whole machine unusable. Have never encountered such a situation before - on either Apple or IBM PCs.
A RAM upgrade can not render the system unusable or DEAD. There was something else wrong with that Mac from the start and putting the RAM in it made it show faster then it would of without the extra RAM. Even if you ibnstalled the Wrong RAM the system shouldn't fail completely. Installing the wrong RAm would just stop the system from booting. Once removing the bad or wrong RAM the system would be back to normal.