It's whisper quiet. You can hear it if the room is silent, but there's no way you could say it was loud.
iMac's are great, but... The display will outlast the computer by years. I bought this 27" last month and before that I was using a 23" that I bought in 2004. Back then the top spec iMac was a 1.8 G5. You'd not want to be using that now! There's also a ton of room in a Mac Pro to upgrade and expand. I've changed the video card, added more RAM (there's 8 slots) and I'm running five hard drives (two of which are SSD) and a Blu-ray drive.
The Mac Pro in the pics I got in June 2008 to replace a Powermac 2x2.5. It's liquid cooling sprang a leak, out of warranty but Apple kindly replaced it with this Pro. Aside from RAM and drives, I've not paid out for a new computer since 2004, so my initial £2.5k for the Powermac is starting to look like a bargain now!