I don't have a good feeling about using any repair facility other than one authorized by Apple. Too many horror stories about botched repairs and call backs. I suggest going to the Apple store and using their flat rate repair option. I believe (could be wrong - so check first) their flat rate repair fee is $325 or maybe a bit more. They'll send it out to an Apple repair depot. Takes about a week.

Of course you need to weigh that against buying a new iMac. You may not be ready to pay that much. Tough decision either way.

I take it you're not in the US... I'm not sure what the flat rate repair fee or if they even do that outside the US. So be sure to call first.