Another note about switching crank sizes -- your gain ratios will not change much in lower gears, but will change a lot in higher gears. If your favorite gear right now is 50 x 17 for example (gain ratio of 6.1), on a 160 mm crank the equivalent gain ratio of 6.1 is 50 x 16. Your gain ratio will be pretty much the same in lower gears. At the high end, the gear ratio for 50 x 12 will be 8.7 on 160 mm cranks vs. 8.2 for 170 mm cranks. I don't think this is a bad thing -- you usually use the highest gears downhill, and you won't spin out as fast with higher gain ratios in your "downhill" gears.