With the amount of typing I do, it would be nice to have a mechanical keyboard. If you haven't heard that term before, a mechanical keyboard is one that has individual switches for each key. Most keyboards that either come with a desktop machine, or are for sale at your local big-box retailer, are not individual mechanical switches, but instead have a rubbery dome under each letter that comes in to contact with a circuit when you push the key. The tactile differences are easily noticeable when you compare the two side-by-side. Some mechanical switches give an audible "click" when the keypress registers, some just give a bit of tactile feedback. That feedback, combined with the increased stability of the keys can lead to faster and surer typing.
Out of the models I've seen for sale right now, I thing that the Das Keyboard Model S Ultimate Silent Mechanical Keyboard would be my favourite, seeing as how the "Datahand" input device isn't being made any more.
Update: I just bought this keyboard! It feels a bit strange, but there is no doubt that I will be faster and more accurate than I would be with my old Logitech keyboard. If you're a serious computer user and you're looking for a real keyboard, I suggest getting one that uses mechanical switches. The one I got uses the Cherry MX Brown switches, so there isn't a loud click but there is still a satisfying keyboard sound when the key bottoms out.