They won't make a big difference to the overall power, maybe pick up a couple of bhp, but the effect on throttle response is very noticable! For a 900 go for 41s, most people recomend 39s for the 600 / 750 engine.
Source - Chris Kelly at California Cycleworks in the states, he supplied mine, here in 3 days and cheaper than UK suppliers.
Oh, and fuel consumption improves a little, which helps pay for them! :-)
http://www.ca-cycleworks.com/