If you are smart you can push the envelope and go for a mega wide range gearing option by purchasing a (9 speed) 29'er cassette; ripping it apart to replace the 34T sprocket with the 36T.
Now if you want to be sneaky, you can mix and match sprockets between the 11-34 cassette and the 29'er cassette to create you own custom ratios.
The most important thing is not to mix up the 8 speed and 9 speed (plastic) spacers, because that will cause all sorts of problems.
I use this principle to give the following numbers: 11T - 13T - 16T - 20T - 24T - 28T - 32T - 36T
You must use two smallest sprockets from the 8 speed cassette group, as the 13T has a built in spacer.
The reason why you can mix and match 8 speed and 9 speed sprockets is because the sprocket thickness is virtually identical between 8 and 9 speed systems - where they differ is in the cassette spacer thickness; 8 speed being thicker than 9 speed.
This also gives rise to another benefit of 8 speed, in that you can use 9 speed chain on an 8 speed system, thereby giving greater chain side clearance between the next larger adjacent sprockets; reducing ghost shifting from any misalignment issues or aggressive lateral chain whip (which can cause the rear derailleur to move against the sprung pantograph action) when travelling over rough surfaces.
The difference between running 9 speed chain on a 9 speed system and running 9 speed chain on an 8 speed system is the difference between hell and nirvana.
Once you've tried it, you'll never go back.
