In reply to Fraser:
As a pianist, I think forearm support is something you actually do not want. I have worked in many offices, as a computer programmer, and tried a variety of chairs and desks. This is how I would design my ideal computer workstation:
Your chair should be at a height so that your feet are flat on the floor and it should support your lower back. It should not have arm rests and should not support your shoulders. If it pivots at all, the spring should be extremely stiff so that it provides comfort by softening the support without letting you fall backwards. (I think this is fairly conventional theory.)
Your monitor should be at a height so that, looking straight ahead, horizontally, it does not require you to crane your neck to see any part of it. It should be perfectly vertical, not tilted.
Your keyboard should be low, such that your forearms are horizontal with elbows are at your sides, like a piano. Your mouse should also be at this height. Your palms should be hovering for ideal touch-typing and the backs of your hands should be as flat as possible.
In general, I find that office desks are way too high for me, requiring me to either make my chair too high for proper posture or having my arms uncomfortably sprawled across the desk (I'm only 1.62 metres but that's not abnormally short) and monitors are invariably too close to the desk height, so that if the desk is low enough, they have to be tilted upwards and you are staring downwards with your shoulders hunched. (EDIT: The conventional solution to this is a higher chair combined with a foot-rest, and a pile of phone-books below the monitor. This is a rubbish solution because foot-rests are awful - they prevent you from moving your feet outside of a tiny area and invariably fail to support a more "male" stance)
Post edited at 14:32