Why the blatant consumerism? Will a new Mac Pro make you happy? Is it intrinsically fulfilling? No. It isn't. I shall provide you the specifications, and you will buy a Mac Pro, but immediately give it to me. That will free you from this endless cycle. Lift your capitalist yoke, and I will bear the burden!
It would really depend on what you plan to use FCP for, I would think?
If you're serious about color,etc, the screen on the iMac probably won't suffice. Otherwise, a lot of ram and as fast processor as possible? Is FCP capable of utilizing all eight cores of a full specked Mac Pro? Research!
No not really, it's not a very well optimized program, however Motion, Compressor and Color are much better at utilizing all the power at their disposal. I have a base model and a top of the range model, both with the ATi graphics. Day to day there is not much difference between them while editing.
The real difference comes when transcoding and encoding and rendering your Color timelines. The important thing is to look at what applications your using and what sort of footage you will be working with.
Go with a MacPro. At first I went with an iMac 24. It kept me happy for about 8 months. Then I got into some really big projects and needed more power and storage. Plus with the MacPro you'll have more configuration options. Currently my MacPro has a CalDigit Raid card to handle the I/O date of the hard drives. This speeds the drives up and free's up the CPU. In bays 1 thru 4 I have a 1.5TB drives. Then in the second 5.25 bay I put my system drive. Plus with the card I have external storage options available to me. The iMac or MacBook Pro has very little room for growth. If your serious about the work your doing be serious about your tools. Then again I go into editing as a hobby and ended up as a career.