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I use a Microsoft Natural Ergonomic Keyboard 4000 (w/ a Razer DeathAdder Mouse) for desktop use, as well as an Apple wired k/b with numpad for my Macbook Pro when it is sitting at my desk. I've considered the Logitech DiNovo Edge Mac edition, but I haven't actually used it.

I can't do serious keyboarding on a non-natural keyboard.

I've heard good things about the Logitech G-series gaming keyboards.
 
Ever since Logitech discontinued the S530 I have been searching a keyboard for my Macs that resembles a "real" keyboard with full-throw keys that are shaped like keyboard keys and dished like keyboard keys and resemble something on which people would expect to be able to type 100 wpm (not that the S530 was all that, but it was close and had other nice features).
People archive about 30% more WPM on the flat keys of the new aluminum keyboard, because of the lesser travel of the keys.
Let me put it this way – on my PC I use a 1984 IBM Model M PS/2 keyboard that weighs about 15 lbs and has keys that stand up, looking for a punch. I remember the days when keyboards were function over form…
You probably want the Apple Extended Keyboard II with an ADB-to-USB adapter then. That is what you could call Apple's Model M.
 
Maybe you just need to get used to new keyboards, I've been using these new Apple keyboards on Macs and PC's and they're just great, I am way faster than on those hard ones

However, Apple made some big hard keyboards 3 years ago, you might find them on ebay
 
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