Personally, I will never ever again trade on a Windows based machine. *nix only - in reality, that means OS X or Linux.
At the moment, I trade from a Macbook Pro (hooked up most of the time to a 24 inch display).
The only problem is that good charting applications are still rare (read: non-existent). However, you can always run them inside a virtual machine, while running your trading platform under OS X. With this, you get the best of both worlds.
I think the new iMac is a great deal. The only thing that holds me back is the glare display. I hope that the glare display fad disappears in the near future.