Most regular rear derailleurs can handle it and you'll have more oomph going down the hills and an easier time going up. Assuming you can find that in a 9 speed. If you can't, see if you can get a 12-27. Obviously won't be as big of a difference but I had one before moving to 11-28 and appreciated having 1 more tooth than a 26. It may be tough finding nine speed gear out there. This is what eventually pushed me to a 10 speed.