Steve H's free tutorial:
explains that Logic in 64-bit mode can address a 'virtually' unlimited amount of ram... But the plug-ins need to be 64-bit optimized!
In regards to your 'which Mac' question:
It's a difficult decision to make...
Can you tell me what kind of performance you are experiencing now with your current iMac and what you hope to achieve?
How many instances of your instruments in a typical project? DSP plugins? How many tracks does a typical project you create require? Are you a fan of freezing/bouncing tracks?
I should mention at this point, that afaik older iMacs with 2 firewire ports actually ran them both on a single firewire bus internally anyway. So, having one FW800 port is not a biggie for most people.
External hard drives are pretty cheap and even more convenient than extra drives within a Mac Pro - so that doesn't have to be an issue either...
If money is no object, there is no doubt I would get a fully decked out Mac Pro. It's great value when you consider you'll probably have it as your main Mac for 5 years or more. It will have a good re-sale value should you wish to upgrade earlier.
Otherwise, for my fairly modest needs without needing extra PCI cards the top-end iMac is very tempting indeed. However, I'd prefer a matte screen.
At the end of the day it's a decision you've got to be happy with. I always find these decisions easier to make by seeing the choice in front of me. Why not pop along to your local Apple Store or Retailer and check out the iMac and Mac Pro. You should also be able to load up Logic on both and try them out (albeit without 3rd party plugins).
By the way, while I've got your attention ;) any chance you would like to do our Logic Benchmark test I've posted in the sticky at the top of the thread. Would be interesting to see what you get now and what you get on your new iMac/Mac Pro.