From the stats page CSV:

  19.28% 1366x768
  18.6%  1024x768
  12.95% 1280x800
  7.49%  Other
  7.48%  1280x1024 
  6.6%   1440x900
  5.09%  1920x1080
  3.83%  1600x900
  3.63%  1680x1050
  + others from the csv file below 3%

That resolution is going to dominate for a while, until retina -style screens make an influence on the mass market.

I recently moved from 1280x800 to 1440x900 myself.