Originally Posted by mycoatl
You likely need to stick with 7 sp because of your hub spacing. However, you can also use a 9 sp casette with one of the cogs removed and run what Sheldon Brown calls "8 of 9 on 7." Check
this out for more info about this conversion (it's toward the bottom of the page).
If you do this you will need 9-speed shifters to index properly and you just lock out one click with the derailleur limit screws.