It looks like they also have a very neat and clear way to measure and visualize the speed of PyPy across version: http://speed.pypy.org/overview/ -- didn't someone say that what you can measure you improve?
It would be useful what the memory usage is like to see if there's a speed/memory tradeoff between PyPy and C-Python.
It's complicated :) PyPy's objects are significantly smaller than CPython ones, however the the assembly the JIT generates, plus the bookeeping and profiling data it stores take up space.