The iMac isn't anything to sneeze at. Based off performance reviews I've read and the Intel hardware I've had access too, I would not be surprised if the 2.16GHz model is just as quick as my Dual 2.3GHz G5 tower. There are other advantages to buying the Mac Pro however.
1. More processing power. The additional chip will result in quicker operations in multi-threaded apps (PS, LR, etc) and multi-tasking. It won't be double the speed certainly (or even a quarter), but it will be noticeable.
2. Ram. The iMac is limited to 3GB. The Mac Pro can hold 16GB (which is insane). The Mac is considerably more efficient at using Ram than Windows and you will benefit from having more than 3GB of ram. The 5.5GB I have in my system handles both PS, Lightroom and whatever other apps I have going quite well.
3. Hard drive storage. The iMac will require external drives while you can fit three additional drives in the Mac Pro.
4. The Mac Pro has Firewire 800 at twice the transfer rate of 400. That is fairly much just as quick as an internal drive.
Oh, a note on external drives on the mac. Don't buy USB 2.0 if you can. Get Firewire, it's faster, maintains transfer rates more reliably and OS X just works more friendly with it. It drives me nuts each time I turn on my USB 2 drive on my mac. Spotlight always wants to index it and it can't be stopped which makes backups even slower.
5. If you want to use dual-monitors you can buy matching displays (both size and resolution) for the Mac Pro more easily than the iMac. As mentioned, the iMac can do dual-displays but with a $20 adaptor.
The question is when New Mac pros will arrive. Usually the towers are announced at WWDC (developers conference) in June so if you want to wait keep in mind you may just end up waiting until then. If you need a system more quickly than June, then just buy what you can now and don't worry about it. You'll get your money's worth regardless.
If you choose the Mac Pro, go for the 2.66GHz model. The $300 you save going with the 2.0 is not worth the drop in performance and the increase of $800 is not worth the additional performance of 3.0GHz. That $800 would be better spent on RAM.
My guess is that this release will probably be synchronized with that of of 10.5 (Leopard) that will offer native 64 bit support
Tiger currently has 64-bit support actually. It's just limited to system-level functions. 10.5 will be the first time it'll be available to third-parties. However, Adobe won't be utilizing it until probably CS 4. In reality all it means is that third-party software will be able to address more than 3GB of ram. Any performance difference other than that won't be big enough for anyone to really notice.
I made the switch from 12+ years on Windows (starting with DOS 6 on a 66MHz 486 DX/2) to Macs just over a year ago and I have no regrets doing so. I think the best way to describe a Mac is that it does not get in your way. It just works as they say. If you have any questions on what to expect in the switch feel free to ask. I'd be happy to share what I've experienced.
