No, unless they changed something with the latest version of the iMac, only the Mac Pro uses ECC RAM, but they use the same EFI for all systems, so that's probably why you see something about ECC being on/off. If the system required ECC RAM and you put in non-ECC RAM, it probably wouldn't boot, and the same if you put ECC RAM into a non-ECC RAM system.
Your problem sounds more like a hard drive issue. Start up Disk Utility and make sure to verify the filesystem, which should also trigger SMART checks. And if you didn't get the 3 year AppleCare warranty package, now might be a good time to do that. Especially if the first year hasn't lapsed yet. Haven't had to do it myself, but I understand there's considerably less hassle that way.
And I don't work for Apple except in the sense that I'm an Apple Certified Macintosh Technician (ACMT). That's why I recommend those AppleCare warranties. RAM upgrades on the Intel iMacs are easy, but everything else requires taking out the display, and that's untempered glass on the front of that thing, which is held in by about 15 or so magnetic clips around the perimeter. You have to use a set of suction cups to get the thing off, and then one wrong move and it'll shatter into a million little shards. Now if an ACMT like me breaks that, you get a new one courtesy of Apple. You break it, you're on your own.
I recently upgraded my RAM from 4 to 6 GB on my iMac. I've noticed that the start-up and shut-down time are slower and occasionally my mac will freeze on the desktop a minute before giving me mouse control. When I check the memory bio it tells me that ECC is disabled. Could this be the issue? If so, is it because of the new RAM?