The biggest factors in RSI tend to be taking frequent breaks and making sure you desk and chair are set at proper heights, but after doing those things I was still having frequent wrist pain and decided to look into more ergonomic devices.
I started with the MS 4000 and the Kinesis Vertical Mouse (http://www.kinesis-ergo.com/evoluent.htm), but after a few weeks was still having pain and it was starting to get worse. Your mileage may vary, but I found the MS 4000 to be just about the same level of comfort as a standard keyboard. It didn't really offer anything more than having a bit of a split in it. The vertical mouse seemed to help on days when I was using the mouse a lot, but on days with lots of typing, I was still getting pain.
I decided to bite the bullet and get the more expensive Kinesis Advantage. After getting used to it, my pain went away, so I'd say it was worth the extra cost. Maybe it's just the sunk cost of relearning to type on a weird shaped keyboard makes me want to feel like the time was worth it - but I suppose it doesn't matter as long as the pain is gone. Also, it's just really cool looking which is good for nerd cred in the office. :D