To reply to some comments above:
There are things that need crazy CPU speeds, and there are things that need crazy GPU speeds. Most of them are not the same thing. For gaming, it's all about GPU. For Video conversion, Photoshop, code development, movie making, etc, the CPU is king. I plan on using the Mac for all things CPU related.
The screen in this computer is ungodly good. For gaming, I will use my high end Windows 7 gaming rig hooked up to the Computer. Aside from spending a $1,000 or the screen by itself, there is no other way to produce the visuals this screen can produce. With the PC hooked into the iMac, I just hit command-F2 to toggle between the Windows 7 computer and the iMac.
My laptop has an ATI X1600 GPU with 256 Meg. Oddly, it's the first and last laptop from Apple that will natively drive a dual-link DVI monitor. If I want to use this Dell 30" with any Laptop Apple has ever sold aside from this exact model, I need a $99 adapter that's a little flaky. You can drive a 2560x1600 30" monitor with a displayport from any new Macbook, just not one with only a Dual-Link DVI input.
It does not matter what video card a Mac Pro has in it with respect to gaming. If you bought a Mac Pro to Game, you bought the wrong computer.
To the comment about a large screen effecting gaming performance, the screen in the iMac is smaller in both physical size and resolution then the monitor I am typing this on right now. The Dell is 16x10 aspect ratio, and the iMac is 16x9. Both are 2560 across, but due to the change in aspect ratio, the iMac losses 160 pixels from time to botom (1440 vs 1600). It's a small price to pay for the quality of image improvement. I can't wait to see what games are going to look like in this screen! :)
