A Mac Pro is a far better investment than an iMac, and at $1799, you just can’t get better computing value in a Mac. 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.
Sure, the 3.33GHz 6-core model is faster (see review), but not as much as you might think, and it’s also more than twice the price. For the money you save, you can buy a very nice color calibrated monitor, with money left over towards memory and drives. Users with really demanding needs should stick to the 6-core (or 12 core), but the needs of the vast majority of users are well served by the 4-core 2.66GHz.
Click to see the deal at B&H, and be sure to get AppleCare also, for 3 years of warranty at a price well below what Apple charges. Add in FREE shipping and FREE Parallels 6, and you should feel stoked.
If you want a little faster machine at still a good price, see the 2.93GHz Mac Pro option.