As long as you pickup a desktop external HD and not a laptop external drive, it will be the same hard drive found in your iMac. It's the same 3.5" HD drive, just in a sleek case with the USB or Firewire ports in the back.
The main difference in a desktop and laptop external is physical size. Well, laptop drives are slower and more expensive. Most all externals are designed for desktops because they also work with laptop, just the laptop versions are more portable.
The WD drive in the in the previous post is good. I have an upgraded version of that drive. It came with Firewire 400 and 800 ports. It's more expensive but firewire is faster than USB 2.0.
Hope this helps. (My advise really is to pickup a firewire drive) I would try and find one with a fan as well. It's louder but cooler.
Try tigerdirect.com. I bought a 250GB HD meant for desktop computers and bought a case for $75 total. My WD HD was $150 for a same size drive.
Peace,
Bryan