This is a reprint because it’s well worth considering, and has been a popular choice. See also Cameras, Computers or Taxes.
I’ve long maintained that a Mac Pro is a far better investment than an iMac, and you can get a brand-new Mac Pro at a savings that gets you most of the distance to buying the new 27" Apple Display (or a good way towards my greatly preferred NEC 27" display).
Get this 2.66GHz Mac Pro. It comes with free FREE SHIPPING and several rebates for other stuff. You might be able to find a similar deal at Amazon. It’s brand-new, and eligible for AppleCare just like any other Mac Pro. See Buying a Mac Pro.
With the money you save, you can afford to soup it up as an MPG Pro Workstation; memory and drives and an SSD matter a whole lot to balanced performance. Have it setup and pre-tested, or see Upgrading Your Mac and do it yourself.
It’s a 5% slower clock speed than the current 2.8GHz model (which costs $600 more), but otherwise it’s every bit as good a machine, and the very same model that 90% of my consulting clients have been delighted with over the last 18 months.