Also, remember that the amount of dish in a rear wheel plays a big role in its potential reliability. If you are willing to use a 7spd freehub body, you "gain" about 5mm in the relative positions of the flanges to the center of the axle, which is significant. You can still have an 8-speed setup by doing the "8 of 9 on 7" setup as described in Sheldon's article about
Shimano cassettes (http://sheldonbrown.com/k7.html).